Estimate how fast heat moves through a window. Pick the window type or enter a U-factor, set the size and the indoor and outdoor temperatures, and get the heat-loss rate.
Heat Loss Rate
— BTU/hr
Steady-state conductive loss at the entered temperature difference.Usage Tip
Lowering U-factor and sealing air leaks both cut heat loss — a leaky high-efficiency window can still perform poorly. Check the NFRC label for the whole-window U-factor.
THE MATH
Area (ft²) = (Width / 12) × (Height / 12)
Q = U × Area × ΔT
ΔT = Indoor temp − Outdoor temp
Q = U × Area × ΔT
ΔT = Indoor temp − Outdoor temp
U-factor is the conductive heat transfer rate per square foot per degree F — lower is better. Q is the steady-state heat-loss rate in BTU per hour.
One watt ≈ 3.412 BTU/hr.
One watt ≈ 3.412 BTU/hr.
This is steady-state conductive loss only. It does not include air leakage around the sash, solar heat gain, or framing effects, so real-world performance varies.
U-factors come from the NFRC label on the window; whole-window U-factors run higher than center-of-glass values.
Typical U-factors: single-pane near 1.0, double-pane clear near 0.5, double-pane Low-E near 0.3, triple-pane Low-E near 0.2.
U-factors come from the NFRC label on the window; whole-window U-factors run higher than center-of-glass values.
Typical U-factors: single-pane near 1.0, double-pane clear near 0.5, double-pane Low-E near 0.3, triple-pane Low-E near 0.2.