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Boiler size calculator

Work out the boiler size your home needs from the gas you already use and the shower you already have, not from counting radiators.

Verified: 26 October 2026 from MCS design conditions
How is this calculated?

On your annual statement or your supplier’s app. Not sure? Use a typical figure:

To measure it, hold a bucket under the shower on full for 10 seconds, then multiply the litres by 6.

Showers on the boiler that run at the same time

Leave out electric showers: they heat their own water.

Adjust the assumptions

A typical household’s, by the government’s SAP method. If you know your summer gas use, a summer day’s gas × 365 is a good figure.

The share of the gas that ends up as heat over a year.

Heating, coldest days

2.2 kW

At -3°C outside, as cold as it gets near London Heathrow in all but a few hours a year, heating the way you do now.

Hot water, one shower

29 kW

To heat 12 litres a minute by 35°C, from cold winter mains water to a hot shower.

A combi of about 30 kW covers you. Hot water decides the size: any combi has far more than the 2.2 kW your heating needs. A smaller one still works, but the shower runs cooler or slower in winter.

Even on the coldest days your heating needs less than many combis can turn down to (5.2 to 6.1 kW on the models we checked). On milder days a boiler like that switches on and off rather than ticking over, which wastes a little gas. If you’re replacing a boiler, ask for one with a low minimum output, and turn the flow temperature down either way.

What common combi sizes deliver
Hot water outputLitres a minute at 35°C riseYour showers
24 kW9.8Runs short
28 kW11.5Runs short
30 kW12.3Keeps up
35 kW14.3Keeps up

Your home loses about 119 watts for every degree colder it is outside, worked out from 6,460 kWh of heat a year and 2,255 degree days. Heating that goes off overnight needs extra output to warm the house in the morning, which a combi has plenty of. An installer should still check before fitting a boiler.

Independent calculator. We don’t pass your details to installers. We don’t earn commission from the result. We update underlying rates monthly and source everything from authoritative public data. See the full methodology.

Common questions

What size combi boiler do I need?
For most homes the hot water decides it, not the heating. A combi heats water as it flows, so its size sets how many litres a minute it can make hot: about 9.8 litres a minute from 24 kW and 12.3 from 30 kW, at the 35°C rise manufacturers quote. A 12-litre-a-minute mixer shower needs about 29 kW in winter.
Is a bigger boiler better?
Not for heating. Most homes need far less heat than a combi’s output even on the coldest days, and a boiler that’s too big for the heating switches on and off on mild days rather than ticking over. The combis we checked turn down to between 5.2 and 6.1 kW; a lower minimum suits a home that needs little heat. A bigger combi does give more hot water at once.
Combi or system boiler?
A combi suits a home where one shower or tap runs at a time. If two showers often run together, or you have several bathrooms, no common combi heats enough water at once, and a system boiler with a hot water cylinder, which stores hot water ready, is the usual answer.
Can I trust a sizing from my gas bill?
It measures your home as you heat it, which a rule of thumb can’t. It reads low if you heat only some rooms or keep the house cool, and needs a full year of gas. An installer should do a proper heat loss calculation before fitting anything, and must be Gas Safe registered.

Already have a combi? Two settings cut its gas use without a new boiler: switching off preheat and lowering the flow temperature.