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.
Boiler Size Calculator results
Fill in the details and select Calculate to see your figures.
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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.
How to use the Boiler Size Calculator
- Choose whether you have (or want) a combi, or a system/regular boiler with a hot-water cylinder.
- Enter the number of bedrooms, bathrooms including en-suites, and radiators in the home.
- For a combi, the size follows hot-water demand, so more bathrooms and radiators push you into a higher band.
- For a system or regular boiler, the size follows heat load and cylinder reheat, which tracks bedroom count.
- Treat 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)
Frequently asked questions
What size boiler do I need for a 3-bed house?
A typical 3-bedroom house with around 10-15 radiators and 1-2 bathrooms usually suits a combi boiler in the 28-34 kW band, or a system boiler of roughly 20 kW. The exact figure depends on your hot-water demand and how well insulated the property is, so treat this as a guide and confirm with an installer.
Why is a combi boiler sized differently from a system boiler?
A combi heats water on demand with no stored cylinder, so its kW output has to cover instantaneous hot-water flow, which is why more bathrooms and radiators push it to a higher kW. A system or regular boiler stores hot water in a cylinder, so it is sized by the building's heat load plus the energy to reheat that cylinder, which tracks bedroom count more closely.
Is a bigger boiler always better?
No. An oversized boiler short-cycles, wastes fuel and can wear out faster, while an undersized one struggles to heat the home or deliver enough hot water. The aim is to match the boiler to the home's actual heat load and hot-water demand; bigger is not automatically better.
Does the number of bathrooms really change the boiler size?
For a combi, yes. If two showers might run at once, the boiler needs more kW to deliver enough hot water simultaneously. For a system boiler with a cylinder, the cylinder size matters more than the boiler kW for hot-water performance.
Do I need a heat-loss survey?
For an accurate size, yes. A proper heat-loss survey (to EN 12831 / MCS standards) measures each room's fabric losses to size the boiler correctly. This calculator gives a rule-of-thumb band to start the conversation, but the survey is what your installer should size the boiler from.
Can Tradehand help me get a boiler quote?
Yes. Tradehand is the software UK heating and trade businesses run on, with an assistant that answers every call and message, sends the quotes and books the work, so a local installer can survey your home and give you an accurate boiler size and price quickly. Get a free demo to see how it connects customers to qualified engineers.
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