SettingsSettings for this calculationUS
The air temperature the room is designed to hold on the coldest design day; the help gives each country's figures.
In the UK, the CIBSE Domestic Heating Design Guide (2026), as the MCS heat load calculator applies it: 21 °C (69.8 °F) for living and dining rooms, studies, games rooms and a bedroom with an en-suite; 22 °C (71.6 °F) for bathrooms and shower rooms; 18 °C (64.4 °F) for bedrooms, kitchens, halls, landings, toilets, utility and store rooms in a house built before 2007 (2008 in Scotland), and 21 °C (69.8 °F) in one built since. MIS 3005-D's own Table 1, with a bedroom at 18 °C (64.4 °F), is the floor for a heat pump design. In Canada, the National Building Code's Article 9.33.3.1 asks for heating able to hold at least 22 °C (71.6 °F) in all living spaces. In the US, ACCA Manual J commonly designs every room at 70 °F (21.1 °C), and Section R302.1 of the energy code, as Washington adopts it, caps a heating design at 72 °F (22.2 °C). A metric page opens on 18 °C (64.4 °F), a British bedroom. An imperial page opens on Manual J's 70 °F (21.1 °C).
A design figure for the location from a published table, not the coldest night anyone remembers.
In the UK, MIS 3005-D's Table 2 gives CIBSE Guide A's figure for each weather station, the temperature the outdoor air is warmer than for 99.6% of the hours in a year: Plymouth (27 m, 89 ft) −1.5 °C (29.3 °F), London (25 m, 82 ft) −3.0 °C (26.6 °F), Cardiff (67 m, 220 ft) −3.1 °C (26.4 °F), Belfast (68 m, 223 ft) −3.2 °C (26.2 °F), Manchester (75 m, 246 ft) −4.5 °C (23.9 °F), Birmingham (96 m, 315 ft) −5.1 °C (22.8 °F), Edinburgh (35 m, 115 ft) −5.4 °C (22.3 °F) and Glasgow (5 m, 16 ft) −5.9 °C (21.4 °F). MCS lowers the figure by 0.6 K (1.1 °F) for every whole 100 m (328 ft) the house stands above its station. In the US, use the heating design temperature your state's energy code adopts, or the 99% heating dry-bulb from the ACCA Manual J tables or the ASHRAE Handbook — Fundamentals; Washington's code gives Seattle (SeaTac airport) 24 °F (−4.4 °C). In Canada, the National Building Code takes the figure the authority having jurisdiction sets, or else the January 2.5% value in its Appendix C (Article 1.1.3.1). A metric page opens on London's −3.0 °C (26.6 °F). An imperial page opens on Seattle's 24 °F (−4.4 °C).
Inside the room, plaster to plaster, along one side.
The floor, the ceiling and the room's volume all come from the length, the width and the height. A room that is not a rectangle is best run as two rectangles, each with its own walls that face outside, and the two answers added; a bay can be folded into the length if its glass goes into the window area.
Tools needed: Tape measure
Inside the room, across the other side.
Measured the same way as the length. Which side is called the length does not matter to the floor or the volume; it only decides which of the two wall counts below a wall belongs to.
Tools needed: Tape measure
Floor to ceiling. It sets the room's volume and the height of its external walls.
Under a sloping ceiling, use the average height. The page runs the external walls the full height of the room and counts the roof at the floor's plan area, so for a room in the roof with sloping ceilings, enter the knee-wall height here and scale the roof U-value by the slope's area over the floor's: about 1.4 times at a 45° slope.
Tools needed: Tape measure
How many of the two walls as long as the Room Length have the open air on their far side.
A wall shared with another heated room, or with the house next door, is not external and loses nothing here. A wall onto an unheated garage or porch is safest counted as external, which overstates it a little. A front bedroom in a terrace has one external wall; a corner room in a semi usually has one along each side.
How many of the two walls as long as the Room Width face outside.
Counted as the other pair is. The page multiplies each count by its wall's length and by the ceiling height, then takes the windows and the external door off the total before working the wall's loss.
How readily heat passes through the wall: the lower the figure, the better insulated the wall.
For a UK wall whose build-up nobody has opened, RdSAP 10 (England) gives these. Solid brick built before 1976 goes by its thickness: 2.5 W/m²K (0.44 BTU/hr·ft²·°F) up to 200 mm (7.9 in), 1.7 (0.30) from 200 to 280 mm (7.9 to 11 in), 1.4 (0.25) to 420 mm (16.5 in) and 1.1 (0.19) beyond. A cavity wall as built before 1976 is 1.5 (0.26) and 0.7 (0.12) once filled; as built 1976 to 1982 it is 1.0 (0.18), 1983 to 1995 0.60 (0.11), 1996 to 2002 0.45 (0.079), 2003 to 2006 0.35 (0.062), and 0.26 to 0.30 (0.046 to 0.053) since. A US wall's U-factor is 1 divided by the whole assembly's R-value, studs and all; Washington's energy code sets an above-grade wall's U-factor at 0.056 BTU/hr·ft²·°F (0.32 W/m²K) in Seattle's climate zone, 4C, and that is the figure this field holds for an imperial reader.
All the glazing in the room's external walls, measured over the frames.
Width times height of each window opening, frame and all, added together. A door that is mostly glass belongs here rather than in the door: RdSAP treats a door as a window once 60% or more of it is glazed. The window area comes off the external wall before the wall's loss is worked out.
Tools needed: Tape measure
The whole window's figure, frame included, from its certificate, its label or the defaults in the help.
RdSAP 10's defaults for wood or PVC frames: single glazing 4.8 W/m²K (0.85 BTU/hr·ft²·°F); double glazing fitted in England or Wales before 2002, 3.1 (0.55) with a 6 mm (0.24 in) gap, 2.8 (0.49) with 12 mm (0.47 in) and 2.7 (0.48) with 16 mm (0.63 in) or more; double or triple glazing from 2002, 2.0 (0.35), and from 2022, 1.4 (0.25). Scotland's dates are 2003 and 2023, Northern Ireland's 2006 and 2022, and a metal frame runs higher: single glazing in metal is 5.7 (1.00). A US window's NFRC label prints its U-factor in BTU/hr·ft²·°F, which this box takes as it stands; Washington's energy code sets a window's U-factor at 0.30 BTU/hr·ft²·°F (1.70 W/m²K) in Seattle's climate zone, 4C, the figure this field holds for an imperial reader.
A solid or part-glazed outside door; RdSAP takes a standard one as 1.85 m² (19.9 sq ft).
Leave it at zero for a room with no outside door. A door that is mostly glass goes in the window area instead. The door's area comes off the external wall, as the windows' does.
RdSAP's figure for an outside door in a house built before 2007 is 3.0 W/m²K (0.53 BTU/hr·ft²·°F).
RdSAP 10 Table 26 goes on: 2.0 (0.35) in age band K, 1.8 (0.32) in band L, or 1.6 (0.28) in Scotland, and 1.4 (0.25) from band M, the same as a door onto an unheated corridor or stairwell. A certified door's own figure beats every one of them, and a US door's is on its label.
The ceiling loses heat only when the space above it is colder than the room.
Under another heated room the ceiling loses nothing in this calculation. Under a loft, a flat roof or a sloping roof, the ceiling is an external element with the U-value below. A room under an unheated store, or under the cold roof of a garage, is safest entered as a roof.
Under a loft, read it from the depth of insulation over the ceiling; the help lists RdSAP's figures.
RdSAP 10 Table 16 gives a pitched roof's U-value at the ceiling, with mineral wool or polystyrene at the joists and the loft's own shelter already allowed for: none 2.3 W/m²K (0.41 BTU/hr·ft²·°F), 25 mm (1 in) 1.0 (0.18), 50 mm (2 in) 0.68 (0.12), 100 mm (3.9 in) 0.40 (0.070), 150 mm (5.9 in) 0.30 (0.053), 200 mm (7.9 in) 0.21 (0.037), 270 mm (10.6 in) 0.16 (0.028), 300 mm (11.8 in) 0.14 (0.025) and 400 mm (15.7 in) or more 0.11 (0.019). Where the depth cannot be seen in an older house, RdSAP assumes 0.40, the 100 mm (3.9 in) most lofts have. A flat roof or a sloping ceiling takes its own build-up's figure. Washington's energy code sets a ceiling's U-factor at 0.024 BTU/hr·ft²·°F (0.14 W/m²K) in Seattle's climate zone, 4C, the figure this field holds for an imperial reader.
Decides how the floor loses heat, and which of the floor fields below appear.
A floor on the ground loses heat into the earth, and how much depends on the whole ground floor's size and how much of its edge meets an outside wall, which is why it asks for those. A floor over a garage, an open passage or the outside air is an exposed floor with a U-value of its own.
Added to every element's U-value for heat that leaks round lintels, junctions and frames; 0 if your U-values already allow for it.
The MCS calculator adds RdSAP 10 Table 21's factor to every external element: 0.15 W/m²K (0.026 BTU/hr·ft²·°F) for a house built before 2003, 0.11 (0.019) for 2003 to 2006 and 0.08 (0.014) from 2007. A US U-factor for a wall or ceiling assembly, the kind Washington's code sets, already counts its framing, the studs, plates and joists, so an imperial reader starts at 0; a designer who wants junctions counted on top of that enters an allowance here.
How many times an hour outdoor air replaces the room's air; 0.5 is the design guide's minimum for a room people live in.
The CIBSE Domestic Heating Design Guide (2026) sets 0.5 an hour as the minimum for habitable rooms, and MCS takes 0 for any room with no external envelope: no external wall, window, door, roof or ground or exposed floor, such as an enclosed landing. MCS's own calculator works each room's rate up from the whole house's air permeability, taking BS EN 12831-1's default figure when the house has not been tested, so a room with old sashes or gappy boards can lose more than the minimum. For a tested house, SAP 10.2 divides the air permeability measured at 50 Pa by 20 and uses the result as the infiltration rate in air changes an hour. A chimney, flue or fan is added below.
Each adds a fixed flow of outdoor air from SAP 10.2's table; an open chimney alone takes more heat than all the walls of the room either unit system opens on.
The flows are SAP 10.2 Table 2.1's. A chimney counts as blocked off only while its ventilator is no bigger than 30,000 mm² (46.5 sq in); larger, treat it as open. Only an open flue that takes its air from the room counts: a room-sealed boiler or fire draws from outside and adds nothing, and trickle vents or air bricks on their own are not passive vents. One item per room; for a second, add its flow divided by the room's volume to the air changes.
The water temperature going into the radiator on the design day.
Approved Document L for England (2021 edition, paragraph 5.10) asks for a new or fully replaced wet system to be sized to run at a flow temperature of 55 °C (131 °F) or lower, which is where the page opens in both unit systems. MCS's heat emitter guide grades heat pump designs in flow temperature bands from 35 °C (95 °F) or less up to 65 °C (149 °F), with more stars the lower the band. Enter what the system will run at on the coldest day, because the radiator rating depends on it far more than on the heat loss.
The water temperature leaving the radiator; the drop from flow to return is set by how fast the water is pumped.
The radiator's output follows how far the water sits above the room, and the return pulls that down. Once the return comes back much closer to the room than the flow — below seven tenths of the flow's excess — BS EN 442 takes the logarithmic mean rather than the simple average, and the page switches by itself and says so.
How steeply the radiator's output falls as the water cools; the maker publishes it beside the ΔT50 output.
BS EN 442-2 rates a radiator as a constant times its excess temperature raised to the power n. Purmo's 2024 UK catalogue lists n from 1.2981 for a 300 mm (11.8 in) high single-panel Compact to 1.3600 for a 900 mm (35.4 in) high triple-panel one; 1.3 is the default the Radiator Output (ΔT Correction) calculator uses too. A higher n means a bigger radiator at low water temperatures, so take the figure for the model being bought.
Room design heat loss
1,220 BTU/h
Every element loses its U-value plus the bridging allowance, times its area, times the temperature difference across it. The air that comes in, the room's volume times its air changes an hour plus any chimney, flue or fan, loses 0.33 W for every cubic metre an hour (m³/h) and kelvin of the indoor-to-outdoor difference, which is 1.06 BTU/h for every cubic foot a minute (CFM) and Fahrenheit degree. The radiator rows give the share of its ΔT50 catalogue output a radiator gives at the flow and return entered, and so how many times the room's heat loss its rating has to be; a catalogue rated at another point, such as a US one at an average water temperature, converts on the Radiator Output (ΔT Correction) calculator.
- Lost through the external walls
- 198.35 BTU/h
- Lost through the windows
- 372.6 BTU/h
- Lost through the external door
- 0 BTU/h
- Lost through the ceiling or roof
- 165.05 BTU/h
- Lost through the floor
- 0 BTU/h
- Lost warming the air that comes in
- 487.27 BTU/h
- Room heat loss in watts (W)
- 358.51 W
- Room heat loss in BTU per hour (BTU/h)
- 1,223.27 BTU/h
- Floor U-value used, before the bridging allowance
- 0 BTU/(hr·ft²·°F)
- Room design temperature
- 70 °F
- Indoor-to-outdoor design difference
- 46 Δ°F
- Water temperature above the room (EN 442 excess)
- 52 Δ°F
- Radiator output here, as a share of its ΔT50 rating
- 49.01 %
- Radiator rating needed, as a multiple of the room's heat loss
- 2.04 ×
- Radiator rating to look for at ΔT50, in watts (W)
- 731.49 W
- Radiator rating to look for at ΔT50, in BTU per hour (BTU/h)
- 2,495.94 BTU/h
They open the calculator with your figures already in it
Room Heat Loss and Radiator Size Calculator (Watts and BTU): 1,223 BTU/h — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- BS EN 12831-1:2017, Energy performance of buildings — Method for calculation of the design heat load: each heated space's transmission loss (U × A × temperature difference over every element facing a colder space) plus its ventilation loss, at the internal and external design temperatures
- MCS MIS 3005-D, The Heat Pump Standard (Design), Issue 2.0 (2 December 2024), clause 5.5.1: (a) the heat load to BS EN 12831-1:2017 at internal temperatures not less than Table 1 and external temperatures from Table 2 column A (99%) or B (99.6%), with no uplift for intermittent heating where column B is used; (c) through a solid floor on the ground, the internal design temperature less the local annual average external air temperature; (d) through a suspended floor, the internal less the design external temperature. Table 1: living, dining and bedsitting rooms 21 °C; bedroom, hall and landing, kitchen and toilet 18 °C; bathroom 22 °C. Table 2, from CIBSE Guide A Table 2.5 (altitude, 99% and 99.6%): Belfast 68 m −1.5 and −3.2 °C, Birmingham 96 m −3.2 and −5.1, Cardiff 67 m −1.5 and −3.1, Edinburgh 35 m −3.2 and −5.4, Glasgow 5 m −3.5 and −5.9, London 25 m −1.7 and −3.0, Manchester 75 m −2.7 and −4.5, Plymouth 27 m −0.2 and −1.5
- MCS Heat Load Calculator documentation, Design Conditions: design room temperatures from the CIBSE Domestic Heating Design Guide (2026) Table 2-2 (age bands A to J and K onwards); design external air temperatures from CIBSE Guide A (2015) Table 2.5, lowered 0.6 °C for every whole 100 m the property stands above the reference location (its Glasgow figure is −5.6 °C, against MIS 3005-D Table 2's −5.9 °C); annual mean external temperatures from Met Office records 1981 to 2010, MGD 007 Table B1 (Thames Valley, Heathrow, 11.3 °C)
- MCS Heat Load Calculator documentation, Ventilation Rates: minimum room air change rates from the CIBSE Domestic Heating Design Guide (2026) Table 2-18 — 0.5 an hour for habitable rooms, 0 for an internal room or corridor, and zero for any room with no external envelope area; default air permeability 12 m³/h·m² at 50 Pa from BS EN 12831-1 Table B.6
- MCS Heat Load Calculator documentation, Thermal Bridging: RdSAP 10 Table 21's factor added to all external building elements — 0.15 W/m²K for age bands A to I, 0.11 for J, 0.08 from K
- BRE for the UK government, RdSAP10 Specification, version 4.0 (7 June 2026): Table 1 age bands; Table 3 wall thicknesses; Tables 6 and 13 wall U-values (England); Table 16 roof U-values by loft insulation depth and Table 18 when it is unknown; Table 20 exposed floors; Table 21 thermal bridging; Table 24 windows; Table 26 doors; §3.7, a door taken as 1.85 m²; §5.12, ground floors by BS EN ISO 13370, the U-value rounded to two decimal places, with clay soil (λ 1.5 W/m·K), Rsi 0.17, Rse 0.04 m²K/W, insulation at 0.035 W/m·K, and for a suspended floor a deck of 0.2 m²K/W, a 0.3 m void, 5 m/s wind, shielding 0.05, openings 0.003 m²/m and walls below the deck at 1.5 W/m²K
- BRE for the UK government, SAP 10.2 (14-03-2025): worksheet line (38), ventilation heat loss = 0.33 × air change rate × dwelling volume; Table 2.1 ventilation rates — open chimney 80 m³/h, chimney or open flue to a closed stove 10, to a solid-fuel boiler 20, to another heater 35, chimney permanently blocked 20, open flue under 200 mm 20, intermittent extract fan 10, passive vent 10, flueless gas fire 40; an air permeability at 50 Pa divided by 20 as the infiltration rate
- BS EN 442-2, Radiators and convectors — Test methods and rating: output = K × ΔT^n, rated at 75 °C flow, 65 °C return and 20 °C air (ΔT50); the logarithmic mean excess where (return − room) ÷ (flow − room) is below 0.7
- Purmo, product range GB catalogue (July 2024), Compact panel radiators: outputs at ΔT50 and ΔT30 tested to EN 442, and the n-coefficients from 1.2981 (Type 11, 300 mm high) to 1.3600 (Type 33, 900 mm) — a Type 22 750 × 1100 mm panel, n 1.3460, gives 2,261 W at ΔT50 and 1,137 W at ΔT30
- Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), paragraphs 5.8 and 5.10: heating sized on an appropriate heat loss calculation, and a new or fully replaced wet heating system sized to run at a maximum flow temperature of 55 °C or lower
- Approved Document L, Volume 1: Dwellings, 2026 edition (England), taking effect on 24 March 2027 for building work that is not higher-risk building work and on 24 September 2027 for higher-risk work: paragraph 4.9, heating designed on the BS EN 12831-1 heat loss calculation, and its note, a room-by-room calculation when a new heating system including the heating appliance and/or emitters is installed and a whole-dwelling one when only the appliance is replaced; paragraph 4.10, the 55 °C maximum flow temperature
- ACCA Manual J (8th edition), Residential Load Calculation — the US room-by-room procedure, named here and not reproduced, which designs every room at one indoor temperature, commonly 70 °F
- NIST Special Publication 811, Guide for the Use of the SI, Appendix B.8: one International Table Btu per hour is 0.2930711 W, the International Table Btu being exactly 1.055 055 852 62 kJ
- MCS 021, Heat Emitter Guide for Domestic Heat Pumps, Issue 2.1 (May 2015): the room heat loss in watts times an oversize factor is the emitter output needed at a mean water-to-air temperature difference of 50 °C; heat pump designs graded by flow temperature band, from up to 35 °C to 61–65 °C, with more stars for lower emitter temperatures
- National Building Code of Canada 2020 (National Research Council), Division B: Article 9.33.3.1, heating able to hold at least 22 °C in all living spaces, 18 °C in unfinished basements and 15 °C in heated crawl spaces at the outside winter design temperature; Articles 9.33.3.2 and 1.1.3.1, the outdoor design conditions from the authority having jurisdiction or else Appendix C's January 2.5% values; Article 9.33.5.1, the capacity of a dwelling's heating appliances determined to CSA F280 with those design temperatures
- Washington State Energy Code, Residential (WAC chapter 51-11R): WAC 51-11R-30200, Section R302.1, heating load calculations at an interior design temperature of at most 72 °F (22 °C); WAC 51-11R-60100, Table RC-1, Seattle (SeaTac airport) outdoor heating design temperature 24 °F; WAC 51-11R-30100, King County in climate zone 4C (marine); WAC 51-11R-40211, Table R402.1.2 for Climate Zone 5 and Marine 4, fenestration U-factor 0.30, ceiling U-factor 0.024, above-grade wall U-factor 0.056 and floor U-factor 0.029
Inputs used
- Room Design Temperature
- 70 °F
- Outdoor Design Temperature
- 24 °F
- Room Length
- 13 ft
- Room Width
- 11.5 ft
- Ceiling Height
- 8 ft
- External Walls Along the Length (0, 1 or 2)
- 1
- External Walls Along the Width (0, 1 or 2)
- 0
- External Wall U-Value
- 0.06 BTU/(hr·ft²·°F)
- Window Area (frames included)
- 27 sq ft
- Window U-Value (whole window)
- 0.3 BTU/(hr·ft²·°F)
- External Door Area (0 if none)
- 0 sq ft
- External Door U-Value
- 0.53 BTU/(hr·ft²·°F)
- What Is Above the Room
- A loft, a roof or the open air
- Ceiling or Roof U-Value
- 0.02 BTU/(hr·ft²·°F)
- What Is Below the Room
- Another heated room (no loss downward)
- Ground Floor Construction
- Solid: concrete laid on the ground
- Ground Floor Area of the Whole House
- 480 sq ft
- Exposed Perimeter of the Ground Floor
- 69 ft
- External Wall Thickness at the Floor
- 9.75 in
- Insulation Across the Ground Floor (0 if none)
- 0 in
- Annual Mean Outdoor Temperature (solid floors)
- 52.34 °F
- Exposed Floor U-Value
- 0.03 BTU/(hr·ft²·°F)
- Thermal Bridging Allowance
- 0 BTU/(hr·ft²·°F)
- Air Changes per Hour
- 0.5
- Chimney, Flue or Fan in the Room
- None
- Radiator Flow Temperature
- 131 °F
- Radiator Return Temperature
- 113 °F
- Radiator Exponent (n)
- 1.3
Intermediate steps
- Lost through the external walls
- 198.35 BTU/h
- Lost through the windows
- 372.6 BTU/h
- Lost through the external door
- 0 BTU/h
- Lost through the ceiling or roof
- 165.05 BTU/h
- Lost through the floor
- 0 BTU/h
- Lost warming the air that comes in
- 487.27 BTU/h
- Room heat loss in watts (W)
- 358.51 W
- Room heat loss in BTU per hour (BTU/h)
- 1,223.27 BTU/h
- Floor U-value used, before the bridging allowance
- 0 BTU/(hr·ft²·°F)
- Room design temperature
- 70 °F
- Indoor-to-outdoor design difference
- 46 Δ°F
- Water temperature above the room (EN 442 excess)
- 52 Δ°F
- Radiator output here, as a share of its ΔT50 rating
- 49.01 %
- Radiator rating needed, as a multiple of the room's heat loss
- 2.04 ×
- Radiator rating to look for at ΔT50, in watts (W)
- 731.49 W
- Radiator rating to look for at ΔT50, in BTU per hour (BTU/h)
- 2,495.94 BTU/h
Confidence note: Every element loses its U-value plus the bridging allowance, times its area, times the temperature difference across it. The air that comes in, the room's volume times its air changes an hour plus any chimney, flue or fan, loses 0.33 W for every cubic metre an hour (m³/h) and kelvin of the indoor-to-outdoor difference, which is 1.06 BTU/h for every cubic foot a minute (CFM) and Fahrenheit degree. The radiator rows give the share of its ΔT50 catalogue output a radiator gives at the flow and return entered, and so how many times the room's heat loss its rating has to be; a catalogue rated at another point, such as a US one at an average water temperature, converts on the Radiator Output (ΔT Correction) calculator.
What this calculation does not cover
- This is a design heat loss: the steady rate the room needs on the design day, built the way BS EN 12831-1 builds it for one room. It is not an energy use or running estimate, and it is not a whole-house figure. A boiler or a heat pump is chosen on the sum of every room, or on a whole-house calculation, never on one room's answer.
- Heat that moves between this room and the spaces beside it is left out. A full room-by-room design also counts the flow through internal walls, floors and ceilings to anywhere held cooler, such as a bedroom at 18 °C (64.4 °F) beside a bathroom at 22 °C (71.6 °F), an unheated hall or a neighbour's empty house. Here those surfaces lose nothing unless they are entered as external.
- Each U-value is the one typed in. The RdSAP 10 figures in the help are what an energy assessor assumes for an age band when the construction cannot be seen, and Washington's are the U-factors its energy code sets for a new house; the real wall, window or loft can be better or worse. On an older house, whether a cavity has been filled decides most of the wall's share.
- A ground floor's U-value is worked by RdSAP 10's form of BS EN ISO 13370 and rounded to two decimals as RdSAP rounds it, with RdSAP's assumptions: clay soil, insulation conducting 0.035 W/m·K (0.24 BTU·in/hr·ft²·°F), and for a suspended floor a ventilated void 0.3 m (11.8 in) deep with RdSAP's standard wind and vent openings. It is worked against MIS 3005-D's temperature difference and without BS EN 12831-1's own correction factors for the yearly swing in outdoor temperature and for groundwater. A basement, ground water close under the floor, or a floor with heating laid in it falls outside that, and a US slab on grade is handled by ACCA Manual J through its exposed edge rather than its area.
- The air term rests on the air change rate typed in, an assumption unless the house has been pressure tested, plus SAP 10.2's fixed flow for one chimney, flue or fan. Mechanical ventilation with heat recovery is not modelled, and nor is the current MCS method of working each room's rate up from the whole house's air permeability.
- Nothing is added for warming the room back up after the heating has been off, for an exposed hilltop, or as a safety margin. BS EN 12831-1 makes a reheat allowance where heating is intermittent, and a designer adds one openly. MIS 3005-D says no intermittent-heating uplift is required when a design uses its 99.6% outdoor temperatures, the column this page's London figure is taken from, and makes no such exemption for its 99% column. This is the continuous loss.
- The radiator rating assumes the emitter follows BS EN 442's power law from its ΔT50 figure, with the exponent entered. Fan convectors, underfloor heating and towel rails do not, and a shelf above a radiator, a cover or long curtains in front of it take away output that no catalogue figure includes.
Add the equipment this sizes
This result is a specification — 1,220 BTU/h — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations16
- BS EN 12831-1:2017, Energy performance of buildings — Method for calculation of the design heat load: each heated space's transmission loss (U × A × temperature difference over every element facing a colder space) plus its ventilation loss, at the internal and external design temperatures
- MCS MIS 3005-D, The Heat Pump Standard (Design), Issue 2.0 (2 December 2024), clause 5.5.1: (a) the heat load to BS EN 12831-1:2017 at internal temperatures not less than Table 1 and external temperatures from Table 2 column A (99%) or B (99.6%), with no uplift for intermittent heating where column B is used; (c) through a solid floor on the ground, the internal design temperature less the local annual average external air temperature; (d) through a suspended floor, the internal less the design external temperature. Table 1: living, dining and bedsitting rooms 21 °C; bedroom, hall and landing, kitchen and toilet 18 °C; bathroom 22 °C. Table 2, from CIBSE Guide A Table 2.5 (altitude, 99% and 99.6%): Belfast 68 m −1.5 and −3.2 °C, Birmingham 96 m −3.2 and −5.1, Cardiff 67 m −1.5 and −3.1, Edinburgh 35 m −3.2 and −5.4, Glasgow 5 m −3.5 and −5.9, London 25 m −1.7 and −3.0, Manchester 75 m −2.7 and −4.5, Plymouth 27 m −0.2 and −1.5
- MCS Heat Load Calculator documentation, Design Conditions: design room temperatures from the CIBSE Domestic Heating Design Guide (2026) Table 2-2 (age bands A to J and K onwards); design external air temperatures from CIBSE Guide A (2015) Table 2.5, lowered 0.6 °C for every whole 100 m the property stands above the reference location (its Glasgow figure is −5.6 °C, against MIS 3005-D Table 2's −5.9 °C); annual mean external temperatures from Met Office records 1981 to 2010, MGD 007 Table B1 (Thames Valley, Heathrow, 11.3 °C)
- MCS Heat Load Calculator documentation, Ventilation Rates: minimum room air change rates from the CIBSE Domestic Heating Design Guide (2026) Table 2-18 — 0.5 an hour for habitable rooms, 0 for an internal room or corridor, and zero for any room with no external envelope area; default air permeability 12 m³/h·m² at 50 Pa from BS EN 12831-1 Table B.6
- MCS Heat Load Calculator documentation, Thermal Bridging: RdSAP 10 Table 21's factor added to all external building elements — 0.15 W/m²K for age bands A to I, 0.11 for J, 0.08 from K
- BRE for the UK government, RdSAP10 Specification, version 4.0 (7 June 2026): Table 1 age bands; Table 3 wall thicknesses; Tables 6 and 13 wall U-values (England); Table 16 roof U-values by loft insulation depth and Table 18 when it is unknown; Table 20 exposed floors; Table 21 thermal bridging; Table 24 windows; Table 26 doors; §3.7, a door taken as 1.85 m²; §5.12, ground floors by BS EN ISO 13370, the U-value rounded to two decimal places, with clay soil (λ 1.5 W/m·K), Rsi 0.17, Rse 0.04 m²K/W, insulation at 0.035 W/m·K, and for a suspended floor a deck of 0.2 m²K/W, a 0.3 m void, 5 m/s wind, shielding 0.05, openings 0.003 m²/m and walls below the deck at 1.5 W/m²K
- BRE for the UK government, SAP 10.2 (14-03-2025): worksheet line (38), ventilation heat loss = 0.33 × air change rate × dwelling volume; Table 2.1 ventilation rates — open chimney 80 m³/h, chimney or open flue to a closed stove 10, to a solid-fuel boiler 20, to another heater 35, chimney permanently blocked 20, open flue under 200 mm 20, intermittent extract fan 10, passive vent 10, flueless gas fire 40; an air permeability at 50 Pa divided by 20 as the infiltration rate
- BS EN 442-2, Radiators and convectors — Test methods and rating: output = K × ΔT^n, rated at 75 °C flow, 65 °C return and 20 °C air (ΔT50); the logarithmic mean excess where (return − room) ÷ (flow − room) is below 0.7
- Purmo, product range GB catalogue (July 2024), Compact panel radiators: outputs at ΔT50 and ΔT30 tested to EN 442, and the n-coefficients from 1.2981 (Type 11, 300 mm high) to 1.3600 (Type 33, 900 mm) — a Type 22 750 × 1100 mm panel, n 1.3460, gives 2,261 W at ΔT50 and 1,137 W at ΔT30
- Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), paragraphs 5.8 and 5.10: heating sized on an appropriate heat loss calculation, and a new or fully replaced wet heating system sized to run at a maximum flow temperature of 55 °C or lower
- Approved Document L, Volume 1: Dwellings, 2026 edition (England), taking effect on 24 March 2027 for building work that is not higher-risk building work and on 24 September 2027 for higher-risk work: paragraph 4.9, heating designed on the BS EN 12831-1 heat loss calculation, and its note, a room-by-room calculation when a new heating system including the heating appliance and/or emitters is installed and a whole-dwelling one when only the appliance is replaced; paragraph 4.10, the 55 °C maximum flow temperature
- ACCA Manual J (8th edition), Residential Load Calculation — the US room-by-room procedure, named here and not reproduced, which designs every room at one indoor temperature, commonly 70 °F
- NIST Special Publication 811, Guide for the Use of the SI, Appendix B.8: one International Table Btu per hour is 0.2930711 W, the International Table Btu being exactly 1.055 055 852 62 kJ
- MCS 021, Heat Emitter Guide for Domestic Heat Pumps, Issue 2.1 (May 2015): the room heat loss in watts times an oversize factor is the emitter output needed at a mean water-to-air temperature difference of 50 °C; heat pump designs graded by flow temperature band, from up to 35 °C to 61–65 °C, with more stars for lower emitter temperatures
- National Building Code of Canada 2020 (National Research Council), Division B: Article 9.33.3.1, heating able to hold at least 22 °C in all living spaces, 18 °C in unfinished basements and 15 °C in heated crawl spaces at the outside winter design temperature; Articles 9.33.3.2 and 1.1.3.1, the outdoor design conditions from the authority having jurisdiction or else Appendix C's January 2.5% values; Article 9.33.5.1, the capacity of a dwelling's heating appliances determined to CSA F280 with those design temperatures
- Washington State Energy Code, Residential (WAC chapter 51-11R): WAC 51-11R-30200, Section R302.1, heating load calculations at an interior design temperature of at most 72 °F (22 °C); WAC 51-11R-60100, Table RC-1, Seattle (SeaTac airport) outdoor heating design temperature 24 °F; WAC 51-11R-30100, King County in climate zone 4C (marine); WAC 51-11R-40211, Table R402.1.2 for Climate Zone 5 and Marine 4, fenestration U-factor 0.30, ceiling U-factor 0.024, above-grade wall U-factor 0.056 and floor U-factor 0.029
Which documents these citations point at
- Approved Documents to the Building Regulations (England) — L (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
Standards referenced: BS EN 12831-1, BS EN 442-2 (European Committee for Standardization, as published in the UK by BSI, European (EN)); ISO 13370 (International Organization for Standardization, International); ACCA Manual J (Air Conditioning Contractors of America, United States); CSA F280 (CSA Group, Canada).
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