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Boiler size calculator - What size boiler do I need?

what size boiler for property

Boiler sizes, or outputs, are measured in Kilowatts, or ‘kW’ for ease.

Boiler PowerFor an interactive way of calculating the size of boiler that your home requires, use our 60 second online quoting tool. You'll see fixed prices on boilers which are the perfect power for your home's needs.

Choosing a suitable size boiler is super important for two key reasons:

#1 - It won’t meet your homes heating requirements if you undershoot it.

An underpowered (undersized) central heating boiler will struggle to meet the heating demands of your home, resulting in a shortfall of heating to your radiators, hot water to your taps/shower or worse – both.

Picture Ronnie Corbett pulling a horse drawn carriage uphill. He’ll try alright, but his tiny body would undoubtably fail him.

#2 - You could easily waste hundreds of £££’s by burning extra gas

Boiler EfficiencyAn overpowered (undersized) boiler won’t provide you with additional benefits and although modern condensing boilers will modulate down to meet the lower demands, you’ll have spent significantly more money on a boiler model that you don’t need.

If the boiler you decide to buy isn't swanky, you'll potentially be wasting a wedge of money on burning unnecessary gas (and giving climate change a boost).

Note: It can be worth considering a higher power (kW) boiler than necessary if you’re planning a home extension that would include additional showers or bath units.


Want an exact price on a new boiler? Use our boiler quote calculator (no personal details needed).

What size combi boiler does my home need?

Boiler Power For HouseWe’ve established you don’t want a boiler that’s too low powered or unnecessarily powerful, for the sake of saving money (and the planet). You need one that’s just right. It’s the Goldilocks conundrum of boilers.

To get a more accurate answer to what size combi boiler your home needs, you can use our quick tool, otherwise here's what size combi boiler your home will need based on its bathrooms (or if you don't like reading, use our online tool):

No. of Bathrooms
or Showers
Boiler Size
1 26kW Viessmann Vitodens 050-W 29kW
2 30kW Viessmann Vitodens 100-W 30kW
3 35kW Viessmann Vitodens 050-W 35kW
3+ 35kW* Viessmann Vitodens 111-W 35kW

* For homes with 3 or more bath/shower outlets that could potentially be used at the same time (particularly very large properties), the boundaries of a combi boiler will be pushed, and a conventional boiler will likely be more suitable (with a hot water tank).

However, the Viessmann Vitodens 111-W is a beastly hybrid – a combi boiler with a built in water tank too that is certainly worth exploring. That's also why it made it onto our top 10 list of the best boiler brands

If you live in an average-sized home, see our guide on choosing the best boiler for a 3 bedroom house

To get a quote on a 111-W hybrid combi boiler, use our online tool.

Check our new boiler costs guide to get an idea of how much it may cost you and if you're interested in paying for your boiler monthly, refer to our boilers on finance guide. Whatever type of boiler you need, Heatable provides. 

What size combi boiler by house type:

Really, you should be calculating boiler size by the number of bathrooms/shower (as above), or, use our online quoting tool.

However, as a rough guide, check out the below:

  Minimum kW Size  
What size boiler for... 1 Bath/Shower  2 Baths/Showers  
2 Bedroom House 26kW 29kW
3 Bedroom House 29kW 35kW
4 Bedroom House 29kW 35kW
5 Bedroom House 35kW 35kW

Not sure of the best model? Read our article on the best combi boiler or go straight to our two favourites:

→ Viessmann Vitodens 050-W Review

→ Viessmann Vitodens 100-W Review

Have a large house with plenty of bathrooms, but have your heart set on a combi boiler?

Viessmann Combi Boiler 111-W

Viessmann has managed to propel us some way into the future and made a hybrid boiler – the Viessmann 111-W.

The Germans have essentially gone full Jurassic Park here, controversially merging the DNA of a conventional boiler and a modern combi boiler to create their very own monster. Viessmann finds a way.

How much are boilers anyway? We’ve broken it down into an article so digestible and moreish, it made half of our readers clinically obese. Read all about new boiler costs

Learn all about combi boilers in our what is a combi boiler guide. 

Want to learn about the future of heating in the UK? Check out the latest innovation of hydrogen boilers.  


Want an exact price on a new boiler? Use our boiler quote calculator (your fixed price will appear on screen).

How to calculate what size system or regular boiler your home needs

Regular (or conventional) and system boilers are also referred to as ‘heat only’ boilers. 

So it’s radiators we’re more interested in here, as opposed to bathrooms.

As a rule of thumb guide, account for:

1.5kW per every radiator in your home + 3kW for your water cylinder.

Or, for ease, use our online tool and you'll also see a range of fixed price quotes, win-win.

Note: Thinking about an immersion heater instead? Read our guide to find out how they compare to combi boilers. 

What size boiler for... Boiler Size Calculation Minimum kW Size Recommended Model
6 Radiators 6x 1.5kW + 3kW 12kW Ideal Logic 12kW
8 Radiators 8x 1.5kW + 3kW 15kW Viessmann Vitodens 100-W 16kW
12 Radiators 12x 1.5kW + 3kW 21kW Ideal Logic 24kW
15 Radiators 15x 1.5kW + 3kW 26kW Viessmann Vitodens 100-W 26kW
20 Radiators 20x 1.5kW + 3kW 33kW Viessmann Vitodens 100-W 35kW


Wondering how much a new boiler may cost? Use our boiler installation cost calculator and if you're wondering how two of the biggest boiler brands compared, check out Worcester Bosch vs Baxi

What about converting? From a conventional to a combi…

Combi boilers are remarkably more efficient, cheaper to run and provide instant hot water without delay from your taps, shower of bath.

It’s no surprise that converting to a combi boiler is the UK’s most common New Year’s Resolution.

It can be quite an expensive job, but, at Heatable, we're offering unbeatable fixed prices on (the right size!) boiler conversions from just £2,495.

Do you have a back boiler and want it replaced with a modern combi boiler? Ideal looking like an ideal choice? Get a fixed online price.


Number of
Boiler Size
1 26kW Viessmann Vitodens 050-W 29kW
2 30kW Viessmann Vitodens 100-W 30kW
3 35kW Viessmann Vitodens 050-W 35kW
3+* 35kW* Viessmann Vitodens 111-W 35kW

If you hadn't already guessed, what size combi boiler you would require if you’re converting from a regular boiler is no different to if you already had a combi boiler…

You’d simply have to take into account your hot water demand as displayed above.

Considering a new boiler replacement, but haven't a clue what size you need? Use our online tool which'll do the hard work for you.

Want to have your boiler moved to a different location? Check out our cost of moving boiler guide. If your boiler's losing pressure get a fixed price replacement, online.

If your radiator is cold at bottom it may be a sign of a faulty boiler valve in which case getting your new boiler replaced may be appropriate, check out the Worcester Bosch Greenstar 30i, which you can purchase through us.

Curious to know which boiler brand performed worst? Find out in our report of the worst boilers

How to get the best price on a new (correct size!) boiler

How? Without boasting, buy from us.

Use this clickable form to get a fixed price on the screen instantly.

We're an online, nationwide boiler installation business with 500+ 5 star reviews from a shed load of happy customers.

Fixed Online Boiler PricesBecause we're online, our overheads are low which means we pass those savings on to our customers, ensuring they're always getting the best deal on a new boiler. Plus, because of the volumes that we install, we're able to access exclusive warranty lengths from our partner manufacturers. For example, we can offer 10 years of protection on the Viessmann 050-W range, whereas your traditional installer can only offer you 7. Boom.

To get the best deal on a new combi boiler, answer some quick online questions and we'll give you an instant fixed price.


More advice

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Get a fixed price on a Which? Best Buy in minutes

Heatable is registered in England and Wales No. 08804726 (VAT No. GB205805920 ). Trading address is 1 Station Road, Great Sankey, Warrington, Cheshire, WA5 1RH. Heatable acts as a credit broker (not as a lender) and offers credit products through a panel of lenders. Heatable is authorised and regulated by the Financial Conduct Authority (reference no. 805259).

[1] Price based on a like for like combi swap in a 2 bed, 1 bathroom terraced home with a standard flue

[*] Installation dates are subject to installer availability

[ยน] only available on combi boiler installations

Financial Support: If you are in receipt of certain benefits, you may be entitled to claim a free boiler grant.