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):

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

System / Regular boiler sizing

System and regular boilers are sized on central heating load (~1.5 kW per radiator) plus a hot water allowance (~3 kW per bathroom). 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%

These are guideline calculations based on UK industry standards. A full heating survey by a Gas Safe registered engineer will account for insulation levels, window types, and building age for a more precise specification.