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
Over a heating season, upgrading windows can save hundreds of dollars in energy costs.
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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