Calculates space-heating heat loss using U-value conduction + ventilation/infiltration.
Tender-stage only. Verify final design to project spec and standards.
For design guidance only. Always verify with a qualified engineer.
About this heat loss calculator
This free heat loss calculator estimates the design heat loss of a room or building so you can size radiators, heat pumps and boilers correctly. It follows the steady-state fabric-plus-ventilation method used by UK building-services engineers and aligned with BS EN 12831.
How the heat loss is calculated
Total heat loss is the sum of fabric and ventilation losses at the design temperature difference. Fabric loss is Σ(U·A·ΔT) through every external element, walls, glazing, roof and floor. Where U is the element U-value, A its area and ΔT the inside-minus-outside design temperature. Ventilation loss is driven by the air-change rate and room volume. The result is reported in watts and as a W/m² intensity for quick benchmarking against typical dwelling and commercial figures.
Frequently asked questions
What design temperatures should I use?
Use the internal comfort temperature for the room (e.g. 21 °C living, 18 °C bedrooms) and the external design temperature for your location (commonly −3 to −5 °C in much of the UK). The larger the difference, the higher the heat loss.
Which standard does it follow?
The method aligns with BS EN 12831 for room-by-room heating load. Always confirm U-values and air-change assumptions and apply professional judgement for the final design.
Is the calculator free?
Yes. It runs entirely in your browser with no sign-up, provided by Ensign for engineers and contractors as a quick design-stage estimate.