Boiler Size Calculator
Estimate the boiler output your home needs in kW.
Estimated boiler output
18kW
- Heating demand
- 15 kW
- Hot water allowance
- 3 kW
Rough estimate only, using the common rule of thumb of 1.5 kW per radiator plus 3 kW per bathroom. It is not a boiler specification.
v1.0 · Reviewed September 2026
How this is worked out
Heating demand is 1.5 kW per radiator. Hot water allows 3 kW for the first bathroom and 3 kW for each additional one, and the two are added.
Worked example
- 10 radiators
- 1 bathroom
15 kW heating plus 3 kW hot water
18 kW
Assumptions and limits
- A rough rule of thumb, not a boiler specification. A Gas Safe registered installer should size the boiler from an actual heat loss calculation.
- Combi boilers are usually sized by hot water flow rate rather than heating demand, so a larger output may be needed for good showers.
- Oversizing wastes fuel through short cycling, so bigger is not better.