Boiler size calculator
Work out roughly what output your house needs, in kW, and whether a combi or a cylinder suits it better. It takes about twenty seconds and asks for nothing but the house.
- ✓No name, email or phone number to see your answer
- ✓Tells you the type as well as the output
- ✓Flags what an installer still needs to check
- ✓Carries your size into the enquiry if you want quotes
What size boiler do I need?
Four questions, no details needed. You will get an estimated output in kW and the type that suits the house.
Bedrooms
BathroomsCount en-suites and shower rooms.
RadiatorsA rough count is fine.
What have you got now?
Answer all four and your estimated size appears here.
How boiler size is actually worked out
Boiler size means heat output, measured in kilowatts, and it answers two separate questions at once: how much heat the radiators need, and how much hot water the taps need. Which of those decides the number depends entirely on the type of boiler.
A combi heats water as it flows, with nothing stored, so hot water demand sets the output and bathrooms are what count. A system or regular boiler fills a cylinder, so hot water is already solved and the radiators set the output instead. That is why the calculator asks about both, and why two houses of the same size can need very different numbers depending on which route they take.
Radiators, not floor area
A radiator count is a better proxy for heat demand than square metres, because it already reflects how the house was designed to be heated.
Bathrooms drive combi output
Every extra bathroom is another potential simultaneous draw, and a combi has to meet them from one heat exchanger.
Insulation moves the answer
Loft and cavity insulation, and newer glazing, all lower heat demand. A house improved since the last boiler went in often needs less output.
Mains flow rate is the limit
A combi can only heat the water reaching it. Poor incoming flow caps performance no matter how many kilowatts are on the badge.
Why a bigger boiler is not a safer choice
It is tempting to treat output as headroom and round up. Oversizing does not work that way. A boiler with far more capacity than the house needs reaches temperature quickly, shuts down, cools, and fires again, over and over. That short cycling is harder on the components than steady running, it burns more gas than a correctly sized unit doing the same job, and the larger boiler costs more to buy.
Undersizing has the opposite failure, and it is the one people notice: radiators that never quite get there on a cold morning, or a shower that runs warm rather than hot. Neither is a good outcome, which is why the number matters and why the range above is worth taking to every installer you speak to. Our guide to boiler sizing goes through the same ground in more detail, and combi versus system versus regular covers choosing the type.
What an installer checks that a calculator cannot
Treat the figure above as the starting point for a conversation rather than a specification. On the survey an installer will work through several things no set of four questions can reach, and any of them can move the answer.
- ✓Heat loss calculationRoom by room, accounting for volumes, insulation, glazing and how exposed the property is. This is the number that decides the specification.
- ✓Mains water flow rateMeasured at the tap in litres per minute. It decides whether a combi is viable at all, particularly with more than one bathroom.
- ✓Gas supply pipeA higher output boiler can need a larger pipe run back to the meter, which on a long run through a finished house is real money.
- ✓Flue route and condensateWhere the flue can terminate, and whether the condensate pipe can reach a drain without a pump.
- ✓The existing systemCylinder position, pipework condition and whether the system needs a chemical flush or a full power flush before the new boiler goes on.
When you do come to compare the numbers, our guide to comparing boiler quotes sets out what a quote should itemise, so three prices can be read side by side.
Frequently asked questions
What size boiler do I need for a 3 bedroom house?
Most three bedroom houses with one bathroom and around ten radiators are served by a 24kW to 27kW combi boiler. Add a second bathroom and it moves up to roughly 28kW to 34kW, because a combi heats water as it flows and two bathrooms can draw at once. The radiator count matters more than the bedroom count, so it is worth counting them before you ask anyone to quote.
What does kW mean on a boiler?
Kilowatts measure heat output, which is the rate the boiler can put energy into your radiators and hot water. It is not a measure of running cost or efficiency. A 30kW boiler does not use more gas than a 24kW one to heat the same house; it simply has more capacity available when it is needed.
Is a bigger boiler better?
No, and oversizing is one of the most common faults in a badly specified quote. A boiler with far more output than the house needs spends its time firing up, satisfying the demand quickly, then shutting down again. That short cycling wears the components, runs less efficiently than a correctly sized unit and costs more to buy in the first place.
Can I have a combi boiler with two bathrooms?
Often yes, but it depends on your incoming mains flow rate rather than on the boiler. A combi can only heat the water that reaches it, so if two showers run at once each gets roughly half the flow. Where both bathrooms are used at the same time, a system boiler with a cylinder is usually the more comfortable answer. Any installer quoting for a combi should measure your flow rate before committing.
Should my new boiler be the same size as my old one?
Not necessarily. Older boilers were frequently oversized, and a house that has since had loft insulation, cavity wall insulation or new windows needs less heat than it did. Replacing like for like on output alone can mean paying for capacity you no longer need, which is why a heat loss calculation is worth more than matching the badge on the old unit.
How accurate is this calculator?
It gives you the range an installer would expect for a house like yours, which is enough to tell whether a quote is in the right territory. It is not a specification. A proper figure comes from a heat loss calculation that accounts for room sizes, insulation, glazing and how exposed the property is, and every installer will carry one out on the survey before fitting anything.