Free Boiler Size Calculator
Work out roughly what kW boiler suits your home. Pick combi or system, then enter bedrooms, bathrooms and radiators for a quick size band.
This boiler size calculator gives you a quick estimate of the kW output that suits your home, using the same rules of thumb UK boiler installers apply. Combi boilers are sized by hot-water demand (radiators and bathrooms), while system and regular boilers are sized by heat load and cylinder reheat (which tracks bedroom count). It is a starting point for conversations with an installer; the definitive size always comes from a proper heat-loss survey.
Boiler Size Calculator results
Fill in the details and select Calculate to see your figures.
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Get startedHow to use the Boiler Size Calculator
- 1Choose whether you have (or want) a combi, or a system/regular boiler with a hot-water cylinder.
- 2Enter the number of bedrooms, bathrooms including en-suites, and radiators in the home.
- 3For a combi, the size follows hot-water demand, so more bathrooms and radiators push you into a higher band.
- 4For a system or regular boiler, the size follows heat load and cylinder reheat, which tracks bedroom count.
- 5Treat the result as a starting band and confirm the exact size with an installer's heat-loss survey.
How the Boiler Size Calculator works
Boiler size follows two different logics depending on type. A combi's kW figure is really its hot-water (DHW) output, so it is banded by how much simultaneous hot water the home needs. A system or regular boiler is sized by the building's heat load plus the energy to reheat the cylinder.
- 3-bed semi, 2 bathrooms, 13 radiators, combi: 13 radiators and 2 bathrooms fall in the 28-34 kW band → ~32 kW.
- 4-bed house, system boiler: 4 bedrooms → ~26 kW to cover heat load and cylinder recovery.
- 1-bed flat, 1 bathroom, 6 radiators, combi: ≤10 rads and 1 bathroom → 24-27 kW band → ~27 kW.
This is a quick estimate only. For a combi, the kW figure is its hot-water (DHW) output, not the building's heat load. The definitive size comes from a proper EN 12831 / MCS heat-loss survey carried out by a qualified heating engineer, so always confirm with one before buying a boiler.
Typical UK benchmarks
| Small home / flat combi | 24-27 kW band for ≤10 radiators, 1 bathroom (Boiler Central) |
| Medium 3-bed combi | 28-34 kW band for up to 15 radiators, 2 bathrooms (Heatable) |
| Large 4+ bed combi | 35-42 kW band for high hot-water demand (The Heating Hub) |
| 4-bed system boiler | ~24-30 kW heat load plus cylinder recovery (The Heating Hub) |
Disclaimer
This tool gives an estimate for general guidance only, based on typical UK figures and the details you enter. It is not a formal quote, survey, or professional advice. Always confirm with a qualified tradesperson before relying on any figure, as regional prices, site conditions, and current standards vary.
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Your assistant sends the quotes, chases the invoices, books the work and keeps your customers updated. The first £10,000 Tradehand collects for a business carries no Tradehand fee. After that, Tradehand earns 5% of collected invoice payments. No monthly fee, setup fee, card fee or contract. One office hire costs about £25,000 a year, whether the invoices land or not.
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