About this calculator

UK combi rule of thumb: 1.5 kW per radiator + 3 kW per bathroom. 24 kW suits a flat / small house; 30 kW a 3-bed semi; 35–42 kW a 4-bed with 2 bathrooms.

Frequently asked questions

What does the Combi Boiler Size Calculator (UK) do?

Recommended combi boiler kW by radiators, bathrooms and property size. UK combi rule of thumb: 1.5 kW per radiator + 3 kW per bathroom. 24 kW suits a flat / small house; 30 kW a 3-bed semi; 35–42 kW a 4-bed with 2 bathrooms.

Is the Combi Boiler Size Calculator (UK) free to use?

Yes. Every calculator on Calcurly.com is free, works in your browser without sign-up, and can be embedded on your own website.

How accurate is the Combi Boiler Size Calculator (UK)?

Results are calculated in real time using industry-standard formulas. Treat the output as a well-informed estimate — always cross-check with local regulations, product datasheets, or a qualified boilers & heating professional before committing to a purchase or a job.

What inputs does the Combi Boiler Size Calculator (UK) need?

Enter values for: Radiators, Bathrooms. Sensible defaults are pre-filled where possible so you can start with a single change.

Can I embed the Combi Boiler Size Calculator (UK) on my own website?

Yes — copy the iframe snippet at the bottom of this page, or use the Embed Generator to customise the size. The embedded version stays in sync with the live calculator on Calcurly.com.