Building Heat Loss Calculator
Add up the heat a building loses through each wall, window, door, ceiling and floor, plus air leakage, at your design temperatures — the classic component method.
Method reviewed October 25, 2026. Version 1.0.0. How it works
Result
- Total heat loss
- 81,639BTU/h
- Total heat loss
- 23.93kW
Total heat loss about 81,639 BTU/h (23.9 kW) at 46 °F temperature difference.
- Conduction (all components)
- 61,767BTU/h
- Air leakage
- 19,872BTU/h
- UA (conduction per °F)
- 1,343BTU/h·°F
Heat loss by component
- 1. Wall
- 22,978BTU/h
- 2. Window
- 20,240BTU/h
- 3. Ceiling / roof
- 18,548BTU/h
Show the working
- 1. Wall: Q = A × ΔT ÷ R2,118 × 46 ÷ 4.24 = 22,978 BTU/h
- 2. Window: Q = A × ΔT ÷ R440 × 46 ÷ 1 = 20,240 BTU/h
- 3. Ceiling / roof: Q = A × ΔT ÷ R1,000 × 46 ÷ 2.48 = 18,548 BTU/h
- Air leakage: Q = 0.018 × volume × ΔT × ACH0.018 × 16,000 × 46 × 1.5 = 19,872 BTU/h
The component method
Heat flows through each part of the building envelope at a rate set by its area, its insulation and the temperature difference across it:
- Conduction: Q = A × ΔT ÷ R, or A × U × ΔT, where U = 1 ÷ R.
- Air leakage: Q = 0.018 × volume × ΔT × air changes per hour, where 0.018 BTU/ft³·°F is the heat needed to warm a cubic foot of air by one degree.
Add the components and the leakage to get the design heat loss: the heating capacity the building needs at the outdoor design temperature.
Checked against a published example
The example values the calculator opens with use the wall, ceiling and air-leakage figures from a Washington State energy factsheet on heat-loss calculation, archived by the Air Infiltration and Ventilation Centre. Its wall example (2,118 ft² of R-4.24 wall at a 46 °F difference loses 22,978 BTU/h) and its infiltration example (a 16,000 ft³ house at 1.5 air changes loses 19,872 BTU/h) are reproduced exactly by this engine in our automated tests.
Getting the R-values right
Use the whole-assembly R-value, not the insulation label. Wood studs, plates and headers bridge the insulation, so a wall’s overall R-value is lower than its cavity insulation’s. Windows and doors are rated by U-factor on their NFRC label; their R-value is 1 ÷ U. Basement walls and slabs lose heat to the ground rather than the outdoor air, and need a different method.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Indoor temperature
- 70 °F
- Outdoor design temperature
- 24 °F
- Number of components
- 3 components
- Component 1: type
- Wall
- Component 1: area
- 2,118 ft²
- Component 1: R-value (whole assembly)
- 4.24 h·ft²·°F/BTU
- Component 2: type
- Window
- Component 2: area
- 440 ft²
- Component 2: R-value (whole assembly)
- 1 h·ft²·°F/BTU
- Component 3: type
- Ceiling / roof
- Component 3: area
- 1,000 ft²
- Component 3: R-value (whole assembly)
- 2.48 h·ft²·°F/BTU
- Heated volume (for air leakage)
- 16,000 ft³
- Air changes per hour (natural)
- 1.5
Result
- Total heat loss
- 81,639 BTU/h
- Total heat loss
- 23.93 kW
- Conduction (all components)
- 61,767 BTU/h
- Air leakage
- 19,872 BTU/h
- UA (conduction per °F)
- 1,343 BTU/h·°F
Total heat loss about 81,639 BTU/h (23.9 kW) at 46 °F temperature difference.
Assumptions
- Steady-state conduction, Q = A × ΔT ÷ R, for each component; air leakage Q = 0.018 × volume × ΔT × ACH (0.018 BTU per ft³ per °F for air).
- R-values are for the whole assembly, including framing and air films — usually lower than the insulation label. For a window or door use 1 ÷ its NFRC U-factor.
Limitations
- Below-grade walls and slabs lose heat differently (to the ground); use a slab-edge or ground-coupled method for them.
- This is a heat-loss calculation only; it does not include solar or internal gains.
Frequently asked questions
What is the difference between R-value and U-value?
R-value is resistance to heat flow; U-value is how easily heat flows. They are reciprocals: U = 1 ÷ R. A window with U-factor 0.30 has an R-value of about 3.3.
What outdoor design temperature should I use?
The 99 % heating design temperature for your location: the temperature the outdoor air stays above for all but about 88 hours of a typical year. ACCA Manual J and ASHRAE publish these by city.
How do I estimate air changes per hour?
A blower-door test is best: natural ACH is roughly ACH50 divided by an adjustment factor that depends on climate and building height. Without a test, about 0.3 is typical of a tight new home and 1.0 or more of a leaky older one.
Is heat loss the same as furnace size?
Heat loss is the output the building needs. A furnace’s input rating is higher by its efficiency: input = heat loss ÷ AFUE. The Furnace Sizing Calculator does that step.
Sources
- Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
- Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
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