Window U-factor, thermal resistance and heat loss
Use the supplied whole-window U-factor, area and temperature difference to estimate steady heat flow. Choose SI or US input units explicitly. The reciprocal resistance and both unit systems are shown together.
Calculator: illustrative inputs — Window U-factor, thermal resistance and heat loss
Use the whole-product rating
Q [W] = U [W/(m²·K)] × A [m²] × ΔT [K]
NFRC whole-window U-factor includes the frame and spacer. A centre-of-glass value is not interchangeable with the complete window rating. This calculation does not derive a certified U-factor from glazing layers, and it excludes air leakage and solar gains. Those require separate inputs such as leakage data and SHGC.
Worked examples and units
US U-factor 0.30, area 20 ft² and a difference of 40 °F give 240 Btu/h, about 70.34 W. The equivalent SI U is about 1.7035 W/(m²·K). A temperature difference of 40 °F is 22.222 K, without an offset. Separately, SI U = 1.2, area 2 m² and ΔT = 20 K give 48 W. These are steady heat-flow examples, not annual energy use.