Why is this not called a Manual J calculator?

Because it is not one, and naming it that would be a misrepresentation. Manual J is a defined room-by-room procedure with specific tables for glazing, orientation, shading, infiltration and duct losses. This is the steady-state equation underneath it applied to the whole house. It will tell you whether a quoted boiler is roughly right; it will not tell you what size radiator the back bedroom needs.

Why does ventilation matter so much?

Because heating outdoor air to indoor temperature costs energy, and a draughty house does it continuously. The breakdown shows ventilation as a share of the total — in an old house it is routinely 40-50%, which is why airtightness work often beats adding insulation for the same money.

Should I size the boiler to this figure?

Close to it, plus an allowance for domestic hot water and for warm-up after setback. What you should not do is add a large arbitrary margin: an oversized boiler cycles, runs below its condensing temperature, and delivers worse efficiency than the label promises. Modern condensing plant performs best when it is running near its calculated load, not far below it.

Where do I get a U-value?

From the assembly. For a UK house whose walls and windows nobody has opened up, RdSAP 10 gives the figures an energy assessor uses in England: single glazing 4.8 W/m²K (0.85 BTU/hr·ft²·°F) in a wood or PVC frame and 5.7 (1.00) in metal; double glazing fitted before 2002 2.7 to 3.1 (0.48 to 0.55) by the width of its gap, 2.0 (0.35) from 2002 and 1.4 (0.25) from 2022; solid brick 2.5 (0.44) up to 200 mm (7.9 in) thick and 1.7 (0.30) from 200 to 280 mm (7.9 to 11 in); a filled cavity 0.7 (0.12) in a wall built before 1976, 0.40 (0.070) from 1976 to 1982 and 0.35 (0.062) from 1983 to 1995; and a wall built since 2007 0.26 to 0.30 (0.046 to 0.053). A US window's NFRC label gives its U-factor in BTU/hr·ft²·°F, and this field takes W/m²K, about 5.68 times that figure. Area-weight them across your envelope — glazing pulls the average up sharply even at modest area.

Plumbing & HVAC

Heating & Cooling Load Screening Calculator

Estimate whole-house heating and cooling loads from envelope areas and U-values — a screening check against a contractor's figure.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Total envelope area

Walls, roof, floor and glazing combined.

Every surface between conditioned and unconditioned space. For a simple two-storey house this is roughly two and a half times the floor area; measure it properly if the shape is unusual.

Average U-value (W/m²K)

Area-weighted average through the whole envelope.

Typical whole-envelope averages: 1.5-2.5 for uninsulated solid-wall construction, 0.5-0.8 for a house built to 1990s standards, 0.2-0.35 for a modern build, below 0.15 for Passivhaus. Glazing dominates the average far more than its area suggests.

Internal volume

Floor area times ceiling height.

The height is the term that gets assumed and should not be: a vaulted or raised ceiling adds volume out of all proportion to the footprint, and volume is what drives the infiltration and air-change part of the load. Treat the result as a SCREEN and nothing more — a real Manual J works from the envelope, its insulation, its glazing and its orientation, and it can differ from a volume-based figure by a wide margin in either direction.

Air changes per hour

Infiltration and ventilation rate.

0.5-1.0 for a reasonably tight modern house; 1.5-3 for an older draughty one; below 0.6 needs mechanical ventilation to stay healthy. A blower-door test gives the real figure — anything else is an assumption, and it can be the largest term in the answer.

Design temperature difference

Internal design temperature minus external design temperature.

For a 21 °C internal target and a −3 °C external design condition, ΔT is 24 K. Use the published design external temperature for your location, not the coldest night on record.

Design heat loss

5,710 W

Medium confidence

Steady-state whole-house loss. Sound as a screening figure and as a check on a quote; not a substitute for a room-by-room calculation, which is what sizes emitters.

Fabric loss: 4,321.11 W
Ventilation loss: 1,386.05 W
Kilowatts: 5.71 kW
BTU/h: 19,473.63 BTU/h
Ventilation as a share of the total: 24.29 %

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Steady-state fabric heat loss: Q = Σ(U × A × ΔT), the basis of every load method including ACCA Manual J and CIBSE Guide A
Ventilation heat loss: Q = 0.33 × n × V × ΔT (W), CIBSE Guide A — 0.33 Wh/m³K is the volumetric heat capacity of air
ACCA Manual J (8th ed.) for the full room-by-room procedure this approximates

Inputs used

Total envelope area: 3230 sq ft
Average U-value (W/m²K): 0.6
Internal volume: 8829 ft³
Air changes per hour: 0.7
Design temperature difference: 43.2 °F

Intermediate steps

Fabric loss: 4,321.11 W
Ventilation loss: 1,386.05 W
Kilowatts: 5.71 kW
BTU/h: 19,473.63 BTU/h
Ventilation as a share of the total: 24.29 %

Final result5,707.16 W

Confidence note: Steady-state whole-house loss. Sound as a screening figure and as a check on a quote; not a substitute for a room-by-room calculation, which is what sizes emitters.

What this calculation does not cover

Whole-house, not room-by-room. It cannot size a radiator or a diffuser, only the plant.
Steady-state: no solar gain, no internal gains from people and appliances, and no thermal mass. In a well-glazed house solar gain materially reduces the heating requirement and materially increases the cooling one.
The air-change rate is an assumption unless it came from a blower-door test, and it is often the largest single term.

Add the equipment this sizes

This result is a specification — 5,710 W — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

QtyItemPrice each ($)

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 2026-08-26 · in the site-wide review of 2026-09-06 · v1.0.0

Regulatory standards & verification citations3

Steady-state fabric heat loss: Q = Σ(U × A × ΔT), the basis of every load method including ACCA Manual J and CIBSE Guide A
Ventilation heat loss: Q = 0.33 × n × V × ΔT (W), CIBSE Guide A — 0.33 Wh/m³K is the volumetric heat capacity of air
ACCA Manual J (8th ed.) for the full room-by-room procedure this approximates

Which documents these citations point at

Standards referenced: ACCA Manual J (Air Conditioning Contractors of America, United States).

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Heating a High-Bay Warehouseuses this calculator
Sizing heat for a tall shed whose doors cycle all day: where the setpoint applies, why the stack effect beats the fabric, and radiant against warm air.
Installing Radiant Floor Heatinguses this calculator
A radiant floor gives up only what its surface temperature limit allows, and spacing, water temperature and finishes all follow from that cap.
Residential Thermal Load Calculation & Design Manualuses this calculator
A component-by-component anatomy of the residential heating and cooling load, and what happens when it meets the equipment catalogue.

Still deciding? Load Calculation vs Rule-of-Thumb Sizing — the factors that actually differ, with no invented prices.

How to calculate heating & cooling load screening in 6 steps

Total envelope areaWalls, roof, floor and glazing combined.
Average U-value (W/m²K)Area-weighted average through the whole envelope.
Internal volumeFloor area times ceiling height.
Air changes per hourInfiltration and ventilation rate.
Design temperature differenceInternal design temperature minus external design temperature.
Design heat lossThe tool computes the design heat loss from those figures and shows the formula, its sources, and a confidence rating alongside it.

Design heat loss by total envelope area

Page defaults, not your figures above.

Total envelope area	Design heat loss (W)
2,000 sq ft	4,062
3,000 sq ft	5,399
4,000 sq ft	6,737
5,000 sq ft	8,075
6,000 sq ft	9,413

Frequently asked questions

Why is this not called a Manual J calculator?: Because it is not one, and naming it that would be a misrepresentation. Manual J is a defined room-by-room procedure with specific tables for glazing, orientation, shading, infiltration and duct losses. This is the steady-state equation underneath it applied to the whole house. It will tell you whether a quoted boiler is roughly right; it will not tell you what size radiator the back bedroom needs.
Why does ventilation matter so much?: Because heating outdoor air to indoor temperature costs energy, and a draughty house does it continuously. The breakdown shows ventilation as a share of the total — in an old house it is routinely 40-50%, which is why airtightness work often beats adding insulation for the same money.
Should I size the boiler to this figure?: Close to it, plus an allowance for domestic hot water and for warm-up after setback. What you should not do is add a large arbitrary margin: an oversized boiler cycles, runs below its condensing temperature, and delivers worse efficiency than the label promises. Modern condensing plant performs best when it is running near its calculated load, not far below it.
Where do I get a U-value?: From the assembly. For a UK house whose walls and windows nobody has opened up, RdSAP 10 gives the figures an energy assessor uses in England: single glazing 4.8 W/m²K (0.85 BTU/hr·ft²·°F) in a wood or PVC frame and 5.7 (1.00) in metal; double glazing fitted before 2002 2.7 to 3.1 (0.48 to 0.55) by the width of its gap, 2.0 (0.35) from 2002 and 1.4 (0.25) from 2022; solid brick 2.5 (0.44) up to 200 mm (7.9 in) thick and 1.7 (0.30) from 200 to 280 mm (7.9 to 11 in); a filled cavity 0.7 (0.12) in a wall built before 1976, 0.40 (0.070) from 1976 to 1982 and 0.35 (0.062) from 1983 to 1995; and a wall built since 2007 0.26 to 0.30 (0.046 to 0.053). A US window's NFRC label gives its U-factor in BTU/hr·ft²·°F, and this field takes W/m²K, about 5.68 times that figure. Area-weight them across your envelope — glazing pulls the average up sharply even at modest area.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.