Boiler size calculator for combi and system boilers
Boilers are sold by output in kilowatts, and the right number depends on what the boiler has to do. This boiler size calculator works it out both ways: for a combi boiler, which heats tap water the moment you turn on a shower, and for a system or regular boiler that heats the radiators and a hot water cylinder. It shows the kW, the BTU/h equivalent and every step of the sum.
Space heating can come from your floor area and insulation level, from the radiators you have, or from a heat loss figure you already know. Hot water is worked out from the showers and taps you use at the same time, or from the cylinder size and how fast you want it to recover.
How to use the Boiler Size Calculator
- Choose Combi boiler or System / regular + cylinder.
- Pick how to work out the heating: floor area and insulation, the number and average output of your radiators, or a known heat loss in kW.
- For a combi, list the outlets and how many of each run at the same time — usually the shower plus one tap.
- For a system boiler, enter the cylinder size, storage temperature and reheat time, and whether heating and hot water run together.
- Read the boiler size, check the steps, and copy the summary for quotes.
Boiler Size Calculator features
Combi sizing by flow
Hot water power = flow × 4.186 × temperature rise ÷ 60 — the same physics behind combi flow-rate ratings.
System boiler sizing
Heating plus the power to reheat the cylinder in your chosen time, or the larger of the two with hot water priority.
Three ways to size heating
Floor area with an insulation rule of thumb, total radiator output, or a heat loss you already know.
kW and BTU/h
Every result is shown in kilowatts and BTU per hour for boilers rated either way.
Editable outlets
Add showers, baths and taps with your own measured flow rates.
Why a combi boiler is sized by hot water
A combi has no tank, so it must heat water as fast as it flows. Raising 1 litre per minute by 1 °C takes 4.186 kJ per minute, so kW = L/min × 4.186 × rise ÷ 60. A 9 L/min shower plus a 6 L/min kitchen tap, from 10 °C to 40 °C, needs 15 × 4.186 × 30 ÷ 60 = 31.4 kW — far more than the 6–10 kW most homes need for heating. That is why combis are 24–42 kW while their heating output is often set lower. Turned round, a 32 kW combi delivers about 13 L/min at a 35 °C rise; in winter, when the incoming water is colder, the flow drops.
| Home (rule of thumb) | Typical choice |
|---|---|
| Flat, one bathroom, up to about 10 radiators | 24–27 kW combi |
| House with one bathroom and an en-suite, 10–15 radiators | 28–34 kW combi |
| Larger house, 15–20 radiators | 35–42 kW combi or a system boiler |
| Two or more showers used at once | System or regular boiler with a cylinder |
Sizing a system or regular boiler
With a cylinder, hot water is stored, so the boiler only has to reheat it. Heating 150 litres from 10 °C to 60 °C takes 150 × 4.186 × 50 ÷ 3,600 = 8.7 kWh; recovering that in an hour needs 8.7 kW. If heating and hot water can run together (S-plan or Y-plan controls), add it to the heating load — 6.3 + 8.7 = 15 kW, so an 18 kW boiler. With hot water priority the boiler only needs the larger of the two.
Oversizing is common and wasteful: a boiler much bigger than the heat loss cycles on and off and spends less time condensing. Check that the boiler’s minimum output (its modulation range) is low enough for mild days.
Before you buy
- The incoming mains flow limits a combi: if your kitchen tap gives only 12 L/min with the cold fully open, a bigger combi will not give more hot water.
- Radiator totals at ΔT50 often exceed the real heat loss; the floor-area estimate or a survey is a better guide for the heating side.
- Thinking about a heat pump instead? Try the heat pump size calculator; to see what your current boiler uses, convert meter readings with the gas units to kWh calculator; for electric storage or tankless heaters use the water heater size calculator.
Boiler Size Calculator FAQ
What size boiler do I need?
For a combi, enough kW for the hot water you draw at once — usually 24–35 kW for one or two bathrooms. For a system boiler, the heat loss of the house plus the cylinder reheat, often 12–24 kW. The boiler size calculator works out both from your own numbers.
How many kW combi boiler do I need for 2 bathrooms?
If both showers may run together, about 50 kW of hot water is needed at a 30 °C rise — beyond most combis, so a system boiler with a cylinder is the usual answer. If only one shower runs at a time, a 30–35 kW combi is typical.
How do I convert boiler kW to BTU?
Multiply kW by 3,412: a 30 kW boiler is about 102,000 BTU/h. The calculator shows both.
Is a bigger boiler better?
Not for heating. An oversized boiler short-cycles and runs less efficiently. For a combi, more kW only helps if your mains water flow can supply it.
How many radiators can a 24 kW combi run?
Heating output, not the hot-water rating, decides it. At about 1 kW per radiator at ΔT50, a combi with 18–24 kW heating output covers most homes of up to 15–20 radiators, but the real limit is the heat loss of the rooms.
Is this a substitute for a heat-loss survey?
No. It is an estimate for planning. A Gas Safe engineer should confirm the size, gas supply, flue and water flow before installation.