About the Heat Loss Calculator
Heat loss from a room through fabric and ventilation, and the heating power it needs.
Also known as: heat loss calculation, room heat loss, building heat loss, heating requirement.
How to use this calculator
- Enter wall area, wall u-value, window area, window u-value and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Heat Loss formula
Q = (Σ U×A + 0.33 × ACH × volume) × ΔT
Worked example
Using the values pre-filled in the calculator above:
- Wall area = 40 m²
- Wall U-value = 0.3 W/(m²·K)
- Window area = 8 m²
- Window U-value = 1.4 W/(m²·K)
- Roof or ceiling area = 25 m²
- Roof U-value = 0.16 W/(m²·K)
- Floor area = 25 m²
- Floor U-value = 0.22 W/(m²·K)
the calculator returns:
- Heat loss: 1,155 W
- In kilowatts: 1.155
- In BTU/hour: 3,941
- Fabric loss: 719 W