Heat Loss Calculator
Estimate a building's heat loss in BTU per hour from its exterior surface area, average R-value, and the indoor/outdoor design temperature difference.
How this calculator works
This calculator estimates how much heat a building loses per hour through its exterior surfaces. It converts your average R-value into a U-value, then multiplies that by the building’s exterior surface area and the difference between your indoor and outdoor design temperatures. The bigger any of those three inputs is, the more heat is lost.
Formula: Heat loss (BTU/hr) = exterior surface area × U-value × (indoor temp − outdoor temp), where U-value = 1 ÷ R-value.
Worked example
2,000 sq ft of exterior surface, R-13 average insulation, 70°F indoor design temp, 20°F outdoor design temp:
- U-value: 1 ÷ 13 ≈ 0.077
- Temperature difference: 70 − 20 = 50°F
- Heat loss: 2,000 × 0.077 × 50 ≈ 7,692 BTU/hr
Notes
This calculator uses one blended R-value for the whole building envelope as a simplification. A real heat-loss calculation, like ACCA Manual J, breaks the envelope into walls, windows, roof, and floor, each with its own R-value and area, since windows lose far more heat per square foot than insulated walls do. Blending everything into a single average R-value is a reasonable starting estimate, but it can undercount loss through the weakest parts of the envelope.
How to use
Enter the building’s total exterior surface area, an average R-value for the envelope, and your indoor and outdoor design temperatures. The calculator converts the R-value to a U-value and multiplies through to give an estimated heat loss in BTU per hour, useful as a rough starting point for sizing discussions.
A simplified whole-building estimate. A professional Manual J calculation accounts for windows, air infiltration, and each envelope component separately — confirm equipment sizing with an HVAC contractor.
Frequently asked questions
What is U-value and how does it relate to R-value?
U-value measures how easily heat passes through a material, while R-value measures how well a material resists that heat flow. U-value is simply 1 divided by R-value, so a higher R-value (better insulation) always produces a lower U-value (less heat loss per square foot).
Why does the temperature difference matter so much for heat loss?
Heat loss scales directly with how far apart the indoor and outdoor temperatures are. A 50-degree gap between indoor and outdoor design temperatures loses heat twice as fast as a 25-degree gap through the same wall, since a bigger temperature difference pushes heat out faster.
How is this different from a full Manual J heat loss calculation?
This calculator uses one blended R-value for the entire exterior surface as a simplification. A real Manual J calculation breaks the building envelope into separate components — walls, windows, roof, floor — each with its own area and R-value, because windows lose far more heat per square foot than an insulated wall.
What outdoor design temperature should I use?
Use your region's winter design temperature, not the coldest temperature ever recorded there. HVAC design temperatures are typically based on a percentile of historical winter lows for your area, and a local HVAC contractor or ACCA Manual J reference table can provide the correct figure for your location.
Estimates only. Verify quantities with your supplier before purchasing.