Heat Loss Calculator (U-Value, Walls and Windows)
Enter areas and U-values to estimate how much heat a room or house loses.
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Heat Loss Calculator (U-Value, Walls and Windows) by TheCalcTools
How the Heat Loss Calculator (U-Value, Walls and Windows) works
Heat flows through each surface at a rate of U-value × area × temperature difference. Add every surface to get the fabric heat loss coefficient UA in watts per kelvin.
Multiply UA by the design temperature difference for peak heat loss (to size heaters), or by heating degree days × 24 for yearly energy. Air leakage and ventilation add more loss and are not included.
The formula
Worked example
Using the default values shown in the calculator:
| Wall area | 80 |
|---|---|
| Wall U-value (W/m²K) | 0.35 |
| Roof or ceiling area | 50 |
| Roof U-value | 0.16 |
| Floor area | 50 |
| Floor U-value | 0.25 |
| Window and door area | 20 |
| Window U-value | 1.4 |
| Indoor temperature (°C) | 21 |
| Design outdoor temperature (°C) | -3 |
| Heating degree days per year | 2200 |
- UA = 80 × 0.35 + 50 × 0.16 + 50 × 0.25 + 20 × 1.4 = 76.5 W/K.
- At 21 °C inside and −3 °C outside: 1,836 W. With 2,200 degree days: 4,039 kWh a year.
Result:
| Peak fabric heat loss (W) | 1,836 |
|---|---|
| Total heat loss coefficient UA (W/K) | 76.5 |
| Yearly fabric heat loss (kWh) | 4,039 |
| Share lost through windows | 36.6% |
Frequently asked questions
What are typical U-values?
Modern insulated walls are about 0.2 to 0.35 W/m²K, solid uninsulated brick about 2, double glazing 1.2 to 2.8 and triple glazing about 0.8.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.