Heat Loss Calculator

Estimate how many BTUs per hour a house loses on a cold day, and where the heat is going.

The cold-day temperature for your area.
Walls, Windows and Doors
All exterior walls, before openings.
Whole wall, not just insulation.
From the NFRC label.
Ceiling and Floor
0 for a slab or heated basement.
Air Leakage
Floor area × ceiling height.
Tight new home 0.35; average 0.5–1; leaky 1.5+.
Fractions and decimals both work.
–
BTU per Hour
Temperature difference
Walls
Windows
Doors
Ceiling
Floor
Air leakage
Kilowatts
Furnace at 95% AFUE

Saved Estimates

Items you add are saved in this browser.

BuildingΔTBTU/hrkWRemove

How to Calculate Heat Loss

Heat flows out through every surface in proportion to its area, the temperature difference, and how poorly it insulates. U-factor is 1 ÷ R-value. Air leaking in also has to be heated.

Surface loss (BTU/hr) = area × ΔT ÷ R = area × U × ΔT
Air leakage (BTU/hr) = 0.018 × volume (ft³) × air changes per hour × ΔT
Total heat loss = sum of all surfaces + air leakage

0.018 BTU per cubic foot per °F is the heat capacity of air (0.075 lb/ft³ × 0.24 BTU/lb·°F).

Worked Example

Tips

Use whole-assembly R-values: a 2×6 wall with R-19 batts is about R-15 to R-16 overall once studs and air films are counted. The outdoor design temperature for your area is in ACCA Manual J or the ASHRAE climate tables (the 99% heating design temperature). This is a simplified block load; a full Manual J adds basements, slab edges, duct losses and more, and is what equipment should be sized from.