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

Choosing the right boiler size for your home is crucial for both comfort and energy efficiency. But let’s face it, if you need a new boiler and don’t have the time or mental resources to wade through what could be an awfully boring article, you might just want to get straight to the solution.

You can get a new boiler quote here—it’ll calculate the right size for your property, and we can often fit it in your area as quickly as tomorrow 🤩.

For those sticking around, the size of your boiler depends on various factors like the number of radiators, your home’s insulation, and hot water demand. In this guide, we’ll help you navigate these considerations to ensure your home stays warm and your energy bills stay manageable.

Key Points:

• The boiler size or power output is measured in kilowatts (kW).

• The boiler size you require is largely determined by the number of radiators, bedrooms and bathrooms in your home.

• Combi boilers are generally suitable for small and average-sized homes, while system and regular boilers (boilers with a tank) are more suitable for larger properties with mega hot water requirements 🚿

• To get an accurate calculation of the boiler size that is right for your home, start here. 

What type of boiler is right for your home?

Before considering the size of boiler you need, it’s important to understand the type of boiler that will be most suitable for your property.

There are three main types of domestic boilers used in the UK:

• Combi boilers - these are the most popular type of domestic boiler in the UK, combining heating and hot water into a single compact unit. They are generally suitable for small and average-sized homes.

• System boilers - these have built-in components (e.g., pumps) that reduce the need for a separate cold water tank (like seen in a regular boiler). They require a separate hot water cylinder but no cold water storage tank. They are ideal for homes with higher hot water demands and can maintain water pressure when multiple taps are used simultaneously.

• Regular boilers (heat-only and conventional boilers) - these are essentially an older version of the system boiler. They require a separate hot water cylinder and cold water storage tank and like system boilers are able to provide hot water simultaneously to multiple water outlets. They are most suitable for homes with older radiator systems that may not handle the high pressure from a combi or system boiler.

Why is correct boiler sizing important? 

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

#1 - It won’t meet your home's 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.

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

An 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).

Please 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.

Prefer video? Watch our short video on choosing the right size boiler for your home below: 

What size boiler do I need?

What size combi boiler for my home?

We’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 hot water and heating demands.

* 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.

*Enquire directly with our team for quotes on this model

That's also why it became 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. 

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 it. 

What size combi boiler by house type

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

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

Not sure of the best model? Read our article on the best combi boiler or check out some of our favourites that we have reviewed:

• Viessmann Vitodens 050-W Review.

• Viessmann Vitodens 100-W Review.

• Viessmann Vitodens 111-W Review. 

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

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

Image of the interior of the Viessmann 111-W - showing how it has its very own internal water tank:

Patrick Garner/Heatable

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, that 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. 

What size regular or system boiler?

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 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.

The table below provides a rough guideline for the regular or system boiler size you will require: 

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

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. 

Got a larger home? You may also be interested in our guide on the best 40kW combi boilers. 

Join Thousands of Satisfied Customers:

We’re proud to be Which? Trusted Traders with over 10,000 glowing reviews on Trustpilot. Check out our Reviews and Testimonials page and watch Adrian’s video testimonial below to see why so many trust us for their heating needs.

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 or bath. Your combi boiler will always be a higher rated KW than the equivalent requirement for a regular boiler (boiler with a tank), this is because they need some extra punch to deliver hot water on demand.

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 £2,995. *Monthly payments available 💸

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.

Here are some popular choices of boiler for Heatable customers who’ve changed from a boiler with a tank or back boiler to a combi:

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.

Want to have your boiler moved to a different location? Check out our cost of moving boiler.

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

Go get that yummy, correctly sized boiler 🏃🏻‍♀️

You may need a new boiler.

You may need it fast and with affordable monthly payments.

You may even need it from a company with top-end local engineers, with 10,000 Trust Pilot reviews to back them up. Plus a Which? Trusted Trader award for good measure.

You may need a new boiler quote from us, Heatable 😘