Heat Loss Calculator
Estimate the fabric heat loss from a room.
Estimated heat loss
403W
- Surface area
- 64 m²
- In BTU/h
- 1,376 BTU/h
An estimate of fabric heat loss only. Ventilation losses and thermal bridging are not included.
v1.0 · Reviewed September 2026
How this is worked out
Surface area is the two floor-and-ceiling faces plus the four walls. Heat loss in watts is that area × the average U value × the temperature difference.
Worked example
- 4 m × 3.5 m × 2.4 m
- U value 0.3
- 21°C difference
64 m² × 0.3 × 21
403 W, about 1,376 BTU/h
Assumptions and limits
- Fabric loss only. Ventilation and air infiltration losses are not included and can be a large share of the total.
- Applies one average U value to every surface, including the floor and ceiling, and treats windows and doors as part of that average.
- A room in the middle of a house loses much less than this, because internal walls face heated space rather than outdoor temperature.
- Not a substitute for a room-by-room heat loss survey to BS EN 12831 when sizing a heating system.