Boiler Size Calculator — What kW Boiler Do I Need?

Our boiler size calculator estimates the correct kW output based on your property's hot water demand and central heating load. The method differs depending on the boiler type.

Combi boiler sizing

Combi boilers are sized primarily by hot water demand (number of bathrooms), because a combi heats water on demand and its rating is set by simultaneous hot-water flow rather than radiator count. We use the following industry guideline ranges:

• 1 bathroom: 24–28 kW
• 2 bathrooms: 28–34 kW
• 3+ bathrooms: 34–42 kW (consider a system boiler instead)

A combi quotes two separate ratings for the same unit — a higher domestic-hot-water (DHW) output (the headline kW) and a lower central-heating (CH) output (typically ~80% of the DHW figure, e.g. a 30 kW combi often gives ~24–26 kW for heating). These are not added together. We cross-check the radiator-derived heating load against the combi's CH output and flag a system boiler if the heating demand is too high.

System / Regular boiler sizing

System and regular boilers are sized on central heating load (a rule of thumb of ~1.5 kW per radiator) plus a hot water allowance (~3 kW per bathroom for an indirect cylinder). The total is adjusted by a property type factor — detached homes lose more heat than mid-terrace properties.

Property type adjustment

• Flat: −15% (shared walls reduce heat loss)
• Mid-Terrace: −10%
• End-Terrace: Baseline
• Semi-Detached: +5%
• Detached: +15%

The bathroom ranges, the ~1.5 kW/radiator and ~3 kW/bathroom rules of thumb, and the property-type factors above are industry guideline figures, not a substitute for a calculated design load. Accurate boiler sizing requires a room-by-room heat-loss calculation to BS EN 12831-1 (CIBSE CP1 / the CIBSE Domestic Heating Design Guide describe the UK method). Since Approved Document L came into force on 15 June 2022, Building Regulations require that a dwelling's heating system is sized on a proper heat-loss calculation and is not significantly oversized. A full heating survey by a Gas Safe registered engineer will account for insulation levels, window types and building age for a precise specification.