Boiler Size Calculator
Enter how many radiators and bathrooms the home has and choose the type of boiler. The calculator works out the heating load, the hot water need and the standard boiler size to ask for.
- Combi, system or regular
- Radiators and bathrooms
- Hot water flow
Count every radiator, including towel rails. Count a bathroom for each shower or bath that might be used at once.
Combi and system boilers are sized in different ways. A combi heats water as you use it, so it is sized for the shower. A system or regular boiler heats a stored cylinder, so it is sized mostly for the radiators. That is why a small flat with a combi still needs about 24 kW.
Boiler size chart
Typical homes. Search for a home.
| Home | Radiators | Bathrooms | Combi (kW) | System (kW) |
|---|---|---|---|---|
| Flat | 6 | 1 | 24 | 18 |
| Two-bed terrace | 8 | 1 | 24 | 18 |
| Three-bed semi | 10 | 1 | 25 | 18 |
| Three-bed with en suite | 12 | 2 | 30 | 24 |
| Four-bed detached | 15 | 2 | 30 | 28 |
| Five-bed detached | 20 | 3 | 35 | 35 |
A heat loss survey gives a more exact figure for your home.
Radiators and boiler size
One bathroom. Slide the radiators.
Each small block is one radiator. The boiler shows the standard size to ask for.
How to size a boiler
Heating and hot water. Step through it.
Count radiators
About 1.5 kW each.
Each kW of combi output heats about 0.4 litres a minute of water by 35 °C, so a 30 kW combi gives about 12 litres a minute.
Combi, system or regular
Boiler types. Compare them.
Hot water on demand.
Sealed, with a cylinder.
Tanks in the loft.
Homes with low water pressure may not get a good shower from a combi, whatever its size.
Before getting quotes
Boiler checks. Tick each item.
Tick each item as you go.
A boiler that is much too big turns on and off often, wastes gas and wears out faster.
What you get
Boiler sizing. Each tile explains one result.
Standard kW.
From radiators.
Flow rate.
For your home.
For comparison.
Modern condensing boilers can turn down their output, so a slightly larger combi still runs efficiently at low heat.
Frequently Asked Questions
No question matches that search.
What size boiler do I need?
A combi for a home with one bathroom is usually 24 to 27 kW, and 30 to 35 kW with two. A system boiler is about 1.5 kW per radiator plus 3 kW.
What size combi boiler for a three-bed house?
About 24 to 30 kW, depending on the number of bathrooms.
How many kW per radiator?
About 1.5 kW on average, though large radiators can need 2 kW or more.
Is a bigger boiler better?
No. An oversized boiler cycles on and off and wastes fuel.
When should I choose a system boiler over a combi?
When two or more showers or baths may be used at the same time.
How much hot water does a combi give?
About 0.4 litres a minute for every kW, so a 30 kW combi gives about 12 litres a minute.
What is a heat loss survey?
A room-by-room calculation of the heat each room loses, done by an installer to size the boiler and radiators exactly.
A boiler sized for your home
Count the radiators and the bathrooms, choose combi or system and round up to a standard size. Then ask your installer to confirm with a heat loss survey.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.