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Boiler Size Calculator
Choosing the right boiler size is essential to save energy, ensure your heating system runs efficiently and keep your bills within your budget.
Boiler size means the output in kilowatts (kW), which is the amount of energy needed to provide your home with heating and hot water. If you install a boiler that's too small, it may not provide enough heat and hot water for your family. Buy one that's too large and you're wasting energy and money.
So, how do you find out what size boiler you need before installing a new boiler? Use our free boiler size calculator below to get a quick estimate. It's based on your heating needs and the number of bedrooms, bathrooms and radiators you have.
Find the right boiler size by answering the questions about your home, then select the boiler type you want and click calculate to get your answer.
Why not try it now?
Boiler Size Calculator
Table of Contents
How to Use the Boiler Size Calculator
Getting a boiler size estimate for your home only takes a few minutes.
Simply work through the steps below, and the boiler size calculator will do the rest:
- Home Size — Select the option that best matches your property, from a 1-bed flat up to a large 5+ bed detached house.
- Boiler Type — Choose combi, system or regular/conventional boiler. Each type has a different typical output range, so this helps the calculator narrow down its recommendation.
- Number of Radiators — Select how many radiators are in your home, as this directly impacts the output your boiler needs to provide.
- Hot Water Demand — Choose from normal demand with one bathroom to very high demand for larger families or simultaneous shower use. The higher your hot water demand, the more powerful a boiler you're likely to need.
- Home Insulation — Let us know how well insulated your property is. Older or poorly insulated homes typically lose more heat, so they may need a slightly higher output to compensate.
- Get Your Boiler Size Estimate — Click calculate, and you'll instantly get a recommended boiler size estimate in kW, tailored to your property.
What Size Boiler Do I Need?
As a general guide:
- Terraced houses or flats — 12-24kW.
- Semi-detached or Bungalow — 24-30kW.
- Detached home — 30-40kW.
However, to find the ideal boiler type for your home, you do have to take a number of factors into consideration:
- The type of boiler.
- The number of bathrooms and the hot water demand.
- Number of radiators.
- Property age and home insulation.
If you have underfloor heating or a large family, then you are likely to need a higher hot water demand than the average household, which means you may need a slightly bigger boiler. If you are unsure, then ask a local heating engineer for any advice.
Combi Boiler
Below we take a look at what size combi boiler would be recommended if you were looking at installing a new boiler in the following situations. This is also with the assumption a 3-bed semi-detached (or larger) is likely to have a second toilet or multiple bathrooms that require heating.
| Property Type | Number of Radiators | Recommended Boiler Size |
|---|---|---|
| 1-Bed flat | 5 | 9 to 12kW |
| 2 Bed Terrace | 8 | 12 to 18kW |
| 3-Bed Semi-detached home | 10 to 12 | 18 to 26kW |
| 4/5-Bed detached | 16 to 20 | 27 to 40kW |
Combi boiler size is usually dependent on needing more power than system or regular boilers, largely due to the fact they supply hot water on demand.
System or Regular Boiler
Below we provide recommendations on what size system or regular boiler would be recommended based on property type and number of radiators. Again, this is with the assumption a 3-bed semi-detached (or larger property) has an additional bathroom or toilet that may also require new central heating.
| Property Type | Number of Radiators | Recommended Boiler Size |
|---|---|---|
| 1-Bed flat | 5 | 24kW |
| 2 Bed Terrace | 8 | 27kW |
| 3-Bed Semi-detached home | 10 to 12 | 28 to 34kW |
| 4/5-Bed detached | 16 to 20 | 34 to 43kW |
Boiler Size for Underfloor Heating
Underfloor heating runs cooler than radiators, so it doesn't automatically need a bigger boiler size. Instead, boiler sizing depends on your floor area and how much heat your home loses, not your radiator count.
If your home has both underfloor heating and radiators, select the higher radiator and hot water demand options in our boiler size calculator to cover the combined load.
Boiler Output vs Boiler Size
Just to clarify, by "boiler size" our calculator is referring to the boiler's output energy (measured in kW) and not the physical dimensions of the boiler or the space it requires. As such, our guide relates to how much heating and hot water the boiler is able to produce, rather than its physical measurements.
Having said that, if your system boiler size requires a high energy output, you don't necessarily need one that's going to take up a lot of room. Many modern boilers currently on the market are available as a compact design, often well-suited to properties that don't have much storage room.
Types of Boilers and Typical Sizes
There are three main types of boiler you can choose from. It's important to assess the needs of your household so that your boiler matches those needs.
For example, if you have a large family and people are likely to use the showers at the same time, then you need a boiler that can provide strong water pressure even when more than one shower is being used.
Combi Boilers
Below are some of the key points to consider when choosing a combi boiler:
- On-demand hot water, as the heating and water come directly from the boiler unit.
- Therefore a combi boiler doesn't require a separate hot water tank.
- Energy efficient and space-saving.
- Ideal for smaller homes.
- Typical output — 12-35kW.
System Boilers
- Uses a separate hot water cylinder.
- Meaning you have to wait for the water to heat up.
- Ideal for homes with more than one bathroom.
- Delivers strong water pressure even with simultaneous use.
- Typical output — 18-30kW.
Regular (Conventional) Boilers
- Has a hot water cylinder and a cold-water tank.
- Once the water is heated, it is automatically stored in the cylinder.
- Mostly used in older, larger homes with a standard central heating system.
- Your home needs space for a hot water cylinder and a cold-water tank.
- Best for homes with more than one bathroom in a larger household.
- Typical output — 25-40kW.
How Much Does a New Boiler Installation Cost?
Now you know roughly what size boiler you need, it's worth knowing what you might pay for it.
Boiler prices vary depending on the type, brand and kW output you choose, but as a guide, supply and installation typically costs:
- Combi Boiler — £1,200 to £4,000.
- System Boiler — £1,100 to £4,000.
- Regular (Conventional) Boiler — £1,750 to £5,500.
If gas isn't an option for your home, an oil boiler installation typically costs around £3,300, while a biomass boiler installation costs around £13,000 on average. Both are worth considering if you're off-grid or want a more sustainable way to heat your home.
For a more accurate boiler estimate try our boiler installation cost calculator.
Get Quotes From Local Heating Engineers
Only a qualified Gas Safe registered engineer can install a gas boiler. We can help you find heating engineers in your area to give you free, no-obligation quotes. Enter your postcode to get started. It's free, quick and easy, and you'll be able to compare at least 3 quotes to find the one that suits you best.
Request a quote from local Gas Safe Engineers today.
Last updated by MyJobQuote on 17th August 2026.
