SettingsSettings for this calculationUS
The area of the specific wall, window, roof, or floor surface being analyzed.
Run this once per distinct surface (e.g. once for walls, once for windows, once for roof) and add the results for a total building load — each surface usually has a different U-value.
The insulating R-value of this specific surface assembly.
Use the Thermal Bridging Calculator's effective R-value for framed walls, or a window's NFRC-labeled U-factor converted to R-value (R = 1/U) for glazing.
Your target indoor temperature.
21°C (70°F) is a common heating design target.
Your region's extreme winter (or summer) design temperature, not the average.
Local design temperatures (typically the 99% or 1% annual design condition) are published by ASHRAE and many local building departments — using a typical/average temperature instead will undersize the system for genuinely cold or hot days.
Heat transmission load
944 BTU/hr
This computes conductive transmission loss through one surface using standard physics — a complete Manual J (or Manual N/CIBSE-equivalent) whole-building load calculation also accounts for air infiltration, internal gains, solar gain, and every surface combined.
- U-value
- 0.08 BTU/hr·ft²·°F
- Equivalent in watts
- 276.75 W
They open the calculator with your figures already in it
HVAC Thermal Load (U-Value) Calculator: 944 BTU/hr — 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)
- Standard heat transmission formula: Q = U-value x surface area x temperature difference (ΔT), where U-value = 1 / R-value
Inputs used
- Surface Area
- 220 sq ft
- Assembly R-Value (hr·ft²·°F/BTU)
- 13
- Indoor Design Temperature
- 69.8 °F
- Outdoor Design Temperature
- 14 °F
Intermediate steps
- U-value
- 0.08 BTU/hr·ft²·°F
- Equivalent in watts
- 276.75 W
Confidence note: This computes conductive transmission loss through one surface using standard physics — a complete Manual J (or Manual N/CIBSE-equivalent) whole-building load calculation also accounts for air infiltration, internal gains, solar gain, and every surface combined.
What this calculation does not cover
- Conduction through one surface, nothing else. Air leakage and ventilation, solar gain through glass, internal gains from people, lighting and appliances, latent (humidity) load, and duct or distribution losses are all outside the formula. Run it on every surface and add the results and you still have a transmission subtotal, not a heating or cooling load.
- This is not a Manual J, Manual N or CIBSE-equivalent load calculation and it is not a basis for selecting or sizing equipment. Where a jurisdiction, utility programme or inspector requires a load calculation, it will require a recognised whole-building method, not a per-surface transmission figure.
- The R-value is taken at face value. Thermal bridging through framing and fasteners, gaps and compressed insulation, wind washing at the edges, and the R-value loss some foams show at cold temperatures all put the real assembly below its nominal rating, and none of them are modelled here. Interior and exterior air films are not added either.
- Steady state, and a rate rather than a quantity. It answers what the surface loses at the instant both design temperatures hold; thermal mass, night setback and the daily temperature swing are absent, so the figure cannot be multiplied by hours to get fuel use, runtime or a bill.
- Surfaces that do not face outdoor air are out of scope. A slab, a basement wall or a floor over a crawl space exchanges heat against ground and buffer-space temperatures on a different basis, and the outdoor field clamps to −40 °C to 45 °C (−40 °F to 113 °F), a range that reaches neither a summer attic nor the coldest published design conditions in the far north.
Add the equipment this sizes
This result is a specification — 944 BTU/hr — 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 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Standard heat transmission formula: Q = U-value x surface area x temperature difference (ΔT), where U-value = 1 / R-value
Cite this page
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