Heat Loss Calculator
Use the heat loss calculator to estimate the power of a heater needed to keep your room at a comfortable temperature.
Heat loss4,615.38 BTU/h1,352.6 W · U = 0.07692 BTU/h·ft²·°F
U-value0.07692the reciprocal of R
Temperature difference60 °F33.33 °C
In watts1,352.64 W
Per square foot4.6154 BTU/h
Metric U-value0.4368 W/m²·KR-13 US is RSI 2.289
Over 24 hours110,769 BTU32.463 kWh · 1.1077 therms
If R were doubled to 262,307.69 BTU/hhalving the loss — but the next doubling only halves again, so returns diminish fast
R-value to halve this lossR-26
The formula
Q = U × A × ΔT, with U = 1 ÷ R
Diminishing returns are built into the formula
Because heat loss goes as 1 ÷ R, going from R-5 to R-10 saves as much as going from R-10 to R-infinity. The first few inches of insulation do most of the work, which is why upgrading an uninsulated wall pays back quickly and adding a fifth layer to an already-good roof does not.
This calculation covers conduction through one assembly only. A real building also loses heat through air leakage, thermal bridges at studs and junctions, and ventilation — in a leaky house infiltration can exceed everything the walls lose.