What Size Boiler Do I Need? (kW Guide and Calculator)

Try the boiler size calculator

Enter your bedrooms, bathrooms, radiator count and insulation level below and the calculator suggests a kW band for a combi and for a system or regular boiler. It is a guide, not a survey: your installer should confirm the figure with a heat loss calculation and a mains flow test before you order.

Boiler size calculator

A quick estimate of the output band to discuss with your installer. It is a guide, not a heat-loss survey: a Gas Safe registered engineer should size the boiler properly before you buy.

Boiler type Combi (heating + instant hot water) System or regular (with hot water cylinder)

Bedrooms 12345 or more

Bathrooms and shower rooms 123 or more

Radiators Up to 67-1011-1516-20More than 20

Insulation and age of home Modern or well insulated (post-2000, double glazing, loft and wall insulation) Average (1950s-1990s, some insulation) Older, solid walls or poorly insulated

Suggested output band

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Based on typical UK radiator outputs (about 1-1.5 kW each), hot water demand by bathroom count and a heat-loss allowance for insulation. Combi outputs quoted by manufacturers usually refer to hot water (DHW); the heating output is often lower and modulates down.

The rest of this page explains where those numbers come from, so you can sanity-check any quote you are given and avoid paying for output you will never use. If you are still deciding between a combi, a system boiler and a regular boiler, start with our hub on the different boiler types and come back here once you know the layout.

What do boiler sizes in kW actually mean?

Boiler sizes are quoted in kilowatts (kW), which is the rate at which the boiler can produce heat. A 30 kW boiler can put out up to 30 kilowatts of heat at full tilt, roughly the same as fifteen fan heaters running together. It does not mean it uses 30 kW all the time: modern boilers modulate, turning the flame down to as little as 3-7 kW when the house is nearly warm.

Residential boilers in the UK run from about 12 kW to 42 kW. Combis cluster at 24-35 kW, while system and regular boilers are commonly 12-30 kW. That gap is not because combi homes are bigger; it is because a combi's kW rating is dominated by hot water, as the next section explains.

Heating output vs hot water output: why combis are rated higher

Every combi has two outputs. The heating (central heating) output is what the radiators can draw. The hot water (DHW, domestic hot water) output is the power available to heat mains water instantly as it flows through the boiler. On most combis the headline number in the model name is the hot water figure, and the heating output is lower.

For example, a boiler sold as a 30 kW combi typically offers around 24-25 kW to the radiators and the full 30 kW to the hot tap. A 35 kW model often gives 28-30 kW for heating and 35 kW for hot water. Manufacturers publish both figures on the data sheet, so check them rather than assuming.

Why does this matter for boiler sizing? Because a typical insulated three-bedroom house loses only 8-12 kW of heat on the coldest day. Almost any combi covers that. The reason you choose 30 kW instead of 24 kW is not the radiators but the shower: heating cold mains water from 10°C to 45°C in the time it takes to flow through the boiler needs a lot of power, and more kW means more litres a minute.

System and regular boilers only have a heating output. They warm a cylinder slowly over 20-40 minutes, so they need just a few kilowatts on top of the house's heat loss. That is why a 15-18 kW system boiler is normal in a house where a combi would be 30 kW.

Boiler sizing chart by bedrooms, bathrooms and radiators

The chart below is the quick answer for a reasonably insulated house with double glazing. Add one band if the property is old, solid-walled or has a large open-plan area; drop one if it is a new build with excellent insulation.

So what size combi boiler for a 3 bed house? Usually 28-30 kW with one bathroom, or 33-35 kW if a second shower is used at the same time and the mains can keep up. If you are weighing those exact outputs, our comparison of 24kW, 30kW and 35kW combis goes through the trade-offs model by model, and the best boiler for your home size lists specific models for each house type.

How to calculate boiler size properly

There are two ways to arrive at a kW figure. The rough way is fine for a shortlist; the proper way is what a good installer does before quoting.

The quick radiator method. Count your radiators, allow roughly 1.5 kW for each average-sized panel (more for large doubles, less for small singles), then add about 3 kW if a cylinder has to be reheated. Ten radiators plus a cylinder comes to about 18 kW for a system or regular boiler. For a combi, ignore this result and size on hot water instead, because the heating side will be covered by any model you are likely to buy.

The heat loss calculation. A proper survey works room by room, using floor area, ceiling height, wall construction, glazing, insulation and the design outdoor temperature (around -3°C for most of the UK) to find how much heat each room loses. The totals are added up to give the whole-house heat loss in kW, and the boiler is sized to match with a small margin. Installers follow the method set out in the UK's adopted version of the European standard for heating load, and many use free manufacturer apps to do the arithmetic.

The survey typically lands lower than homeowners expect. A 1930s semi that has had cavity insulation and double glazing often comes out at 9-11 kW, which is why fitting a 35 kW boiler "to be safe" makes little sense unless the hot water demands it.

What flow rate in litres per minute do you need?

For a combi, flow rate is the number that should drive your choice. Hot water output in kW converts to litres per minute at a 35°C temperature rise roughly like this:

A standard mixer shower uses 8-12 litres a minute; a bath tap wants 12-15. The boiler can only heat what the mains delivers, so measure your supply before you buy. Turn the kitchen cold tap on full, time how long it takes to fill a 1-litre jug, and divide 60 by the seconds. Six seconds means 10 litres a minute. If your mains gives 10 litres a minute, a 35 kW combi will produce 10 litres a minute of hot water and no more.

In winter the mains water is colder, so the same boiler delivers slightly less flow at your chosen temperature. Our guide to combi boilers and showers explains what that means for different shower types, and how a combi boiler works covers the plate heat exchanger that does the heating.

Why is a bigger boiler not better?

It is tempting to add a few kilowatts for safety. With a combi it rarely helps and often costs money three times over.

• Purchase price. Within a range, each step up in output typically adds £100-£300 to the boiler price, and the largest models sit in a higher band again.
• Cycling. A boiler cannot run below its minimum output. If the house only needs 4 kW on a mild day and the boiler's minimum is 7 kW, it fires, overheats the water, shuts off and repeats. That short cycling wears the fan, gas valve and ignition parts and keeps the return water too warm for the boiler to condense fully, so efficiency falls.
• No extra comfort. A radiator circuit can only take so much heat. An oversized boiler does not warm rooms faster in any way you would notice; it simply reaches its target and stops.

The exception is a genuine hot water shortfall. If you have the mains flow to support it and two bathrooms in simultaneous use, a 38-42 kW combi earns its price. Otherwise, size for the home you have.

How do you size a system or regular boiler?

Here the logic flips. Hot water comes from the cylinder, so the boiler only needs to cover the calculated heat loss of the house plus a reheat allowance of around 3 kW. The cylinder's size, not the boiler's output, decides how many showers you can run in a row.

As a rule of thumb, allow 35-45 litres of stored hot water per person, so a family of four needs about 150-180 litres, and a two-bathroom house where both are busy in the morning is better with 210-250 litres. Reheat time depends on the cylinder's coil and the boiler output: an 18 kW boiler brings a 180-litre unvented cylinder back up from cold in roughly 30-40 minutes. Our guide to boilers and hot water cylinders covers vented versus unvented cylinders and how to match the two.

A regular boiler for an older house with large cast-iron radiators and single glazing may genuinely need 24-30 kW, which is why the heat loss survey matters more than bedrooms. Fitted prices for either layout are typically £2,000-£4,000 before any cylinder, and our guide to combi and system boiler costs breaks that down.

Get the type right first, the kW second and the brand last. A correctly sized 28 kW combi from a mid-range manufacturer will keep a three-bedroom house warmer, cheaper and longer than an oversized premium model that spends its life cycling.

Frequently asked questions

What size boiler do I need for a 3 bed house?

For a typical three-bedroom house with one bathroom and 8-12 radiators, a 28-30 kW combi is the usual fit, giving around 12 litres of hot water a minute. If the house has two bathrooms used at the same time, step up to 33-35 kW or consider a system boiler with a cylinder. A system or regular boiler for the same house only needs about 15-18 kW because the cylinder handles hot water.

What size boiler for a 4 bed house?

A four-bedroom house with 12-16 radiators and one or two bathrooms usually suits a 30-35 kW combi where the mains flow is strong, or an 18-24 kW system boiler paired with a 180-210 litre cylinder. The second option is the more comfortable choice when two showers run together, because stored hot water does not drop in flow when another tap opens.

How many kW boiler do I need for 10 radiators?

Ten average radiators in a reasonably insulated house add up to roughly 10-15 kW of heat demand, so any combi on the market will cover the heating side; its kW rating is chosen for hot water instead. A system or regular boiler of 15-18 kW is about right, allowing a few kilowatts for reheating the cylinder. A proper room-by-room heat loss calculation gives the exact figure.

Can a boiler be too big for my house?

Yes. A boiler that cannot turn down low enough for your home switches on and off repeatedly, known as cycling, which wears parts and drops efficiency because the return water rarely gets cool enough for full condensing. Larger models also cost more to buy. The right approach is to size a combi for hot water flow and a system or regular boiler for calculated heat loss, not for bedrooms.

Is an online boiler size calculator accurate?

A calculator gives a sound starting point from bedrooms, bathrooms, radiators and insulation, and for most two to four-bedroom homes it will land on the right band. It cannot see your mains flow rate, the condition of your insulation or an unusually large open-plan space, so treat the result as a shortlist. Your installer should confirm it with a heat loss survey and a flow test before ordering.

What size combi boiler do I need for two bathrooms?

If both bathrooms are used at the same time, you need around 15-17 litres of hot water a minute, which means a 38-42 kW combi and a mains supply that can actually deliver that flow. Many homes cannot, in which case a system boiler with an unvented cylinder gives better results. If the second bathroom is rarely used simultaneously, a 33-35 kW combi is usually enough.

Sources and further reading

• Energy Saving Trust - Boilers
• Worcester Bosch - Boilers
• Vaillant UK - Boilers
• Ideal Heating - Boilers
• Heating and Hotwater Industry Council (HHIC)
• Gas Safe Register

Prices and scheme details were checked on 22 October 2026 and are shown as typical ranges. Always get written quotes from Gas Safe registered installers.