About this calculator
Quick radiator sizing in watts and BTU for one room, adjusted for walls, insulation and glazing.
How it’s calculated
Watts = room volume (m³) × a factor for the room’s temperature (35–50 W/m³), × adjustments for external walls, insulation, single glazing and north aspect. BTU = W × 3.412.
Worked example
With these inputs:
- Room length
- 4.5 m
- Room width
- 3.8 m
- Ceiling height
- 2.4 m
- Room
- Living / dining (21°C)
- External walls
- Two
- Insulation
- Average
- Single glazing
- No
- North facing
- No
the result is:
- Heat output needed
- 2,050 W
- Room volume
- 41 m³
- Radiator output at ΔT50
- choose ≥ 2,050 W
Tips from the trade
- Oversized AC short-cycles and leaves rooms clammy, so bigger is not better.
- For heat pumps, a full heat loss calculation is a must and radiators need sizing at the lower flow temperature.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the Building Regulations before ordering.