Window Heat Loss Calculator (U-Factor)
Calculate heat loss through a window using U-factor, window area, and indoor/outdoor temperature difference.
Useful for energy audits and window upgrades.
What Is Window U-Factor?
The U-factor (or U-value) measures how well a window prevents heat from escaping. It is the rate of heat transfer per unit area per degree of temperature difference.
Lower U-factor = better insulation = less heat loss.
This is the opposite of R-value (which measures insulation resistance). U-factor = 1 ÷ R-value.
Heat Loss Formula
Heat Loss (BTU/hr) = U-Factor × Area (ft²) × ΔT (°F)
Heat Loss (Watts) = U-Factor (W/m²·K) × Area (m²) × ΔT (°C)
Where ΔT is the difference between indoor and outdoor temperature.
Typical U-Factor Values
| Window Type | U-Factor (Imperial, BTU/hr·ft²·°F) | U-Factor (SI, W/m²·K) |
|---|---|---|
| Single pane, clear glass | 1.10 | 6.24 |
| Single pane with storm window | 0.50 | 2.84 |
| Double pane, clear glass | 0.49 | 2.78 |
| Double pane, Low-E coating | 0.32 | 1.82 |
| Triple pane, Low-E | 0.20 | 1.14 |
| Best available (triple, gas fill) | 0.10 | 0.57 |
ENERGY STAR requirement for most US climate zones: U-factor of 0.30 or lower.
Practical Example (Imperial)
A 24 sq ft double-pane Low-E window (U = 0.32) on a day where it is 70°F inside and 20°F outside:
- ΔT = 70 − 20 = 50°F
- Heat loss = 0.32 × 24 × 50 = 384 BTU/hr
Compare to a single-pane window (U = 1.10):
- Heat loss = 1.10 × 24 × 50 = 1,320 BTU/hr — 3.4× more heat loss
Practical Example (Metric)
A 2.5 m² double-pane Low-E window (U = 1.82 W/m²·K), indoor 21°C, outdoor −5°C:
- ΔT = 21 − (−5) = 26°C
- Heat loss = 1.82 × 2.5 × 26 = 118 Watts
What this does not tell you
Heat loss per hour is not the same as money saved. Multiply this figure across a heating season and the annual saving from a window upgrade usually lands in the low hundreds of dollars for a whole house, against a replacement bill in the thousands. That gap is why energy payback on new windows runs to decades, and the honest reasons to replace are usually rot, draughts, noise, or windows painted shut rather than the fuel bill. The window replacement payback calculator works that arithmetic through properly.
Two things this calculation ignores. Air leaking around a poor frame often loses more heat than the glass does, so a draughty old window performs worse than its U-factor implies. And a south-facing window earns heat back on sunny days, which the Solar Heat Gain Coefficient measures and this does not.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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