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Construction Calculators

Heat Loss Calculator

Calculate the rate of heat loss through walls and windows using their R-values, area and the indoor/outdoor temperature difference.

sq ft
Typical stud-wall insulation is R-13 for 2x4 walls or R-21 for 2x6 walls.
sq ft
A single-pane window is about R-1; a standard double-pane window is about R-2 to R-3.
°F
°F
Use your local 99% winter design temperature for a realistic worst-case estimate.
Temperature difference —
Wall heat loss —
Window heat loss —
Total heat loss —
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About the Heat Loss Calculator

This calculator estimates conductive heat loss through a building's walls and windows using the standard building-science relationship between R-value (insulating resistance), area and temperature difference. A higher R-value means the assembly loses heat more slowly for the same temperature difference.

Formula: Heat Loss (BTU/hr) = (Area ÷ R-value) × (Indoor Temperature − Outdoor Temperature), applied separately to walls and windows and then added together. This is the same relationship as Area × U-factor × the temperature difference, since U-factor is simply 1 ÷ R-value.

Use your local 99% winter design temperature (available from ASHRAE climate data or a local HVAC contractor) as the outdoor temperature for a realistic worst-case figure, not the record low. This calculator only totals conductive loss through the wall and window area entered — it excludes air infiltration, floors, ceilings and doors, so treat the result as a partial estimate rather than a full Manual J heat loss calculation.

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