Boiler size calculator
Tell us about your home and we will recommend a boiler output in kilowatts (kW), plus whether a combi is the right fit or whether you should look at a system boiler. It uses the standard UK sizing bands, the same logic a good installer starts from, before a full heat-loss survey confirms it.
Find your boiler size in kW
Four quick inputs. We give you a size to put on any quote, and flag when a combi is not the right choice for your home.
Recommended size
kW
Use this number to check a quote. A boiler much larger than this is oversized for your home, ask the installer to justify it with a heat-loss survey.
Based on standard UK sizing bands (Worcester Bosch, Ideal, BestHeating), verified June 2026. A guide only; a Gas Safe heat-loss survey gives the definitive figure for your home.
The standard UK boiler sizes
Combi boilers cluster into three size bands. Match your home to the right one and you avoid both the cold-shower problem of undersizing and the wasted money of oversizing.
| Boiler size | Suits | Radiators | Bathrooms |
|---|---|---|---|
| 24 to 27 kW | Small 1 to 2 bed flat or house | Up to 10 | 1 bathroom |
| 28 to 34 kW | Medium 3 to 4 bed house | Up to 15 | 1 to 2 bathrooms |
| 35 to 42 kW | Large 4+ bed house | Up to 20 | 2+ bathrooms |
For three or more bathrooms, a system or regular boiler with a hot-water cylinder is usually the better choice than a large combi. Source: UK manufacturer sizing guides, verified June 2026.
The belief that bigger is safer
Almost everyone assumes a more powerful boiler is a better boiler, more kW, more heat, more headroom. For boilers, that instinct is wrong, and it costs money. A boiler is not a kettle you want to boil faster. It is a device that runs most efficiently when it runs steadily, matched to your home's actual heat demand.
Oversize it and it does the opposite: it heats the system quickly, hits the target, switches off, cools, and fires again, over and over. This short-cycling burns extra gas, stresses components, and shortens the boiler's life. The "safe" big boiler is actually the expensive one, both to buy and to run.
Why most sizing guides fall short
Most "what size boiler" pages give you a static table and stop there. The table is a fine starting point, it is the one above, but it cannot see your home. Two 3-bedroom houses can need quite different boilers depending on insulation, radiator count and how the bathrooms are used. A table treats them as identical; a real assessment does not.
The deeper gap is that almost no guide explains why the bathroom count, not the floor area, usually decides a combi's size. A combi has no hot-water tank, so it must heat water the instant you turn the tap. Its kW rating is really a measure of how much hot water it can produce at once. That is why a small flat with a powerful shower can need as much boiler as a larger house with a modest one.
The insider view: why installers round up
Here is the incentive most households never see. An installer who fits a slightly oversized boiler almost never gets a complaint, the home is warm and the hot water never runs short. An installer who fits a correctly sized or slightly small boiler risks a callback if anyone feels a cold radiator. So the safe move for them, every time, is to round up.
The cost of "to be safe" Rounding up looks free, but you pay twice: a larger boiler costs more to buy, and an oversized boiler short-cycles and wastes gas for its whole life. The asymmetry is the point, the installer avoids a callback, and you quietly carry the running cost for ten years. This is why you should bring your own kW figure to the quote and ask the installer to justify anything much bigger.
None of this means undersize the boiler, a boiler that cannot meet demand gives cold showers and runs flat out. It means insist on the right size, backed by a heat-loss survey, rather than accepting the comfortable over-spec that suits the installer more than you.
What to actually do
Get your own kW figure first
Use the calculator above before any visit, so you walk into the quote with a number, not a blank page.
Count your radiators and bathrooms
These two figures drive the size more than floor area. Have them ready for the installer.
Ask for a heat-loss assessment
A good installer measures, not guesses. If they only eyeball it and round up, get a second quote.
Challenge an oversized quote
If the recommended boiler is much bigger than your figure, ask why. "To be safe" is not a heat-loss calculation.
The bottom line
Most UK homes need a 24 to 35 kW combi, set mainly by bathrooms and radiators, not square footage. Bigger is not safer, it is more expensive to buy and less efficient to run, because an oversized boiler short-cycles. Take the kW figure from the calculator into your quote, insist on a heat-loss survey rather than a "to be safe" round-up, and you will pay for the boiler your home actually needs, not the one that is easiest to sell you.
Boiler size FAQ
The Selectra expert answers your questions
A typical 3-bedroom house with one or two bathrooms and up to 15 radiators needs a combi boiler of about 28 to 34 kW. Lean to the lower end if the home is well insulated, and the higher end if you have two bathrooms used at the same time. Use the calculator above for a figure tuned to your radiators and insulation.
No, and this is the most common and costly mistake. An oversized boiler costs more to buy and runs less efficiently because it short-cycles, firing up and switching off repeatedly rather than running steadily. That wastes gas and wears the boiler faster. The right size is the smallest that comfortably meets your peak demand, not the biggest you can fit.
For a combi, largely yes. A combi makes hot water on demand, so its output (kW) has to be big enough to run your busiest hot-water moment. One bathroom is comfortable on 24 to 28 kW; two push you to 30 to 35 kW; three or more usually mean a system or regular boiler with a stored hot-water cylinder instead.
A small flat or one-bedroom home with one bathroom and up to about 10 radiators is well served by a 24 to 27 kW combi. Going larger does not give you more heat in a small home, it just costs more and short-cycles. The minimum modern combi output (around 24 kW) is usually plenty.
Treat it as a starting point and ask how they reached it. A good installer does a heat-loss assessment; a rushed one applies a rule of thumb and rounds up "to be safe", which leaves you oversized. Use the kW figure from the calculator to challenge a quote that specifies a much larger boiler than your home needs. A proper survey, not a guess, should set the final number.