Radiator Size Calculator
Estimate the radiator output a room needs in watts and BTU.
Required output
2,016W
- In BTU/h
- 6,879 BTU/h
- Room volume
- 33.6 m³
Approximate sizing estimate using the common watts-per-cubic-metre rule of thumb for a typical insulated UK room.
v1.0 · Reviewed September 2026
How this is worked out
Room volume is length × width × height, multiplied by the watts per cubic metre figure you choose. BTU/h is watts × 3.412142.
Worked example
- 4 m × 3.5 m × 2.4 m
- 60 W/m³ for a lounge
33.6 m³ × 60
2,016 W, about 6,879 BTU/h
Assumptions and limits
- A rule of thumb, not a heat loss calculation. Typical figures are 60 W/m³ for a lounge, 50 for a bedroom and 40 for a kitchen, with more for poorly insulated rooms, large windows or north-facing walls.
- Radiator outputs are quoted at a stated flow temperature, usually delta T 50. A heat pump running at a lower flow temperature delivers far less from the same radiator, so check the manufacturer's output at your actual temperature.