Boiler kW Calculator – What Size Boiler Do I Need?

Last updated: April 29, 2024

What size boiler do you need? Which boiler will provide your home with adequate hot water and heating without wasting energy and driving up your heating bills?

The right size boiler for your home largely depends on your property’s hot water demands but includes other factors such as the size of your property.

In this post, we’re going to help you determine the right power output for your property (measured in kilowatts or kW for short).

Boiler Size Key Points:

  • The boiler size or power output is measured in kilowatts (kW).
  • Factors to consider when determining boiler size include the size of the property, the number of radiators, bedrooms and bathrooms.
  • 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. 
  • A rough guide is to allow 1.5 kW for every radiator and 3 kW for every bathroom.
  • It is also important to consider insulation levels, the age of the property, and the local climate.
  • A professional installer can assess your property’s needs and recommend the best size and type of boiler for your home.

Check out our video on how to vet boiler brands/models in the UK:

What Size Boiler do I need for my Property?

Boiler sizes quoted on various models are not actually dimensions or capacity. The sizing measured in kilowatts is actually referring to the power output that a boiler provides.

Typically, the more heat and hot water needed for the property and household requirements, the higher the kilowatt output, or boiler size will need to be.

Although it may be tempting to go for a boiler size with the highest power output, this may not make sense from a financial perspective as it will likely end up costing you more to run and waste energy you don’t need.

What Size Boiler Do I Need Calculator: 

Boiler size calculator (1)

You can use our free boiler size calculator below to get a rough estimate of what size boiler your house requires:

Factors that Determine the Boiler Size

There are a number of elements that will impact the most appropriate boiler size for the property. These are as follows:

The number of bedrooms

The size of a property to be heated will determine the size of a boiler needed.

The more bedrooms that a property has would indicate a larger size property which would in turn require a higher output boiler.

Check out our quick help guides for choosing a boiler based on your property size: 

The number of bathrooms

As mentioned, typically combi boilers are usually recommended for smaller sized properties with one bathroom.

The pressure of water is also a factor especially when considering a combi boiler as the flow of water will determine how quickly baths are poured or the level of a power shower.

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Should there be more than one bathroom in a property, a combi boiler can struggle to supply the higher flow rate needed, and therefore a hot water cylinder system is often required.

Wondering which boiler manufacturers performed best? Check out our best boiler brands guide for the full rundown and if you are trying to determine who the best boiler installation company is, check out our Boiler Central, Warmzilla and Boxt reviews.

Number of Radiators

The more radiators installed in a property extends the heating system and increases the power needed to supply the system.

A property with lots of radiators and towel rails will require a higher boiler output size to heat the property effectively.

The Boiler Size Rules to Remember:

To determine the exact size boiler you will need i.e. the power output (kW), use the following formula:

Add up the number of radiators in your home, using the following scores:

  • 1 kW for small radiators.
  • 1.5 kW for an average-sized radiator.
  • 2 kW for large radiators.

The total will provide the power output required for your home.

However, if you have a boiler with a hot water storage cylinder/tank you will need to add an additional 3kW to power the tank.

Therefore, if you had 10 radiators of average size and a tank, you would require an 18kW boiler.


1.5kW x 10 radiators = 15kW.

Plus 3kw for the tank.

15kW + 3kW = 18 kW.

Boiler Size Calculator

Taking all of the above information into account, this quick free and easy to use calculator can estimate the boiler size needed for property and advise an indicative cost.

The calculator bases its suggestions on mid-range boilers and average installation costs to provide a rough estimate of the costs involved with installing a new boiler.

Final installed boiler prices will vary depending on the selected brand of boiler, the supplier and installation costs which can also vary depending on the location of the property.

Online boiler suppliers can provide a cost-effective method of sourcing a new boiler and arranging an installation.

How old is your boiler? Should you consider a boiler repair or start looking at the latest boiler prices? It’s no secret that boiler efficiency deteriorates over time.

Other factors to be aware of

Unfortunately, the boiler efficiency advertised on branding or promotional materials may not be achieved once the boiler has been installed into your property due to a number of factors such as:

  • Insulation – Typically, British properties are not well insulated and therefore some energy efficiency will be lost by heat escaping through the property walls or roof. In addition, any gaps in windows, doors or letterboxes can create draughts that heat will escape through.
  • Water Pressure – As discussed, the pressure of water is a factor that determines which time of boiler would be more suitable. Should water pressure fluctuate, the boiler efficiency can be impacted.
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Boiler erp label

What type of boiler do you need? 

When needing a new boiler one of the first tasks in the process is to establish which type of boiler would be most suitable.

Commonly both the property requirements and household usage will determine which type of boiler should be installed.

There are three common types of boiler to choose from when looking to install a new boiler, these are combination (combi boilers), system and regular (heat only) boilers.

Did you know the efficiency of your boiler can impact the amount of energy it uses and ultimately impact your heating bill costs?

Check out our guide to the best condensing boilers if you are interested to find out more. 

You may also find our review of the best eco-friendly boilers of interest.

Combination boilers (Combi boilers)

Also known as combi-boilers, they are the most common type of boiler used in UK homes. They provide both heating and hot water directly from the mains without the need for a water tank.

Not only do combi-boilers help to save physical space without the requirements of large tanks, but also, they can save energy without heating the whole tank, only heating what is required at the point of use.

However, there are limits to combi-boilers as they are typically suited to small to medium-sized properties only, depending on the number of bathrooms and household usage requirements.

Another factor when considering a combi-boiler is the water pressure requirements needed to enable the system to work effectively.

Wondering how you may fund a new boiler installation? Check out our complete guide to boilers on finance, as well as potential boiler grants you may be eligible for. 

Did you know that not only boiler models are wall-hung either, check out our complete guide on floor standing boilers here.

Wondering how much a new boiler costs? Check out our guide on options available if you need a new boiler but can’t afford it, as well as boilers on finance and government boiler grants here.

Regular (or heat-only boilers)

As the name suggests, this type of boiler only provides heat only, rather than heat and hot water like a combi-boiler. Heat only boilers supply the radiators directly via the use of a hot water cylinder.

Hot water is stored in a separate hot water storage tank and water is fed to the system via a cistern tank in the attic.

Regular boilers are the oldest type of residential boilers and so are usually recommended for properties that already use them and those with old pipework.

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Got a common boiler problem? Check out our quick fix guide on boiler lockout, boiler ignition faults, boiler PCB faults, faulty diverter valves and boiler timer issues.

System boilers

A system boiler is a closed system requiring a boiler and a cylinder system. System boilers are essentially a modern version of the regular boiler; as they have more parts internalised, such as the pump.

This means they are quicker to install than regular boilers and take up less space (as they don’t require a cistern tank in the loft).

However, most system boilers also require more space, than more compact boilers such as a combi-boiler.

Since they also take their water directly from the mains water supply, they aren’t always suitable for areas that suffer from low water pressure.

What size boiler is right for your home? If you are thinking about a combi boiler, it is largely determined by the number of radiators in your home. Check out our complete guide to new boiler installation here if you are wondering about the best type of boiler for your home. 

How much may a new boiler cost? Are you considering converting from a conventional to a combi boiler? Use our boiler installation cost calculator to get an estimation.

Combi, regular or system boiler?

So, which type of boiler is the most suitable for your home? These are the general rules:

  • Choose a combi boiler if you have less than 2 bathrooms and/or a small to average size home.
  • Choose a system boiler if you have 2+ bathrooms and/or a medium to large size home.
  • Choose a regular boiler if you already have one installed in your property.

Now, with some background knowledge of the various types of boilers, we can move on to the other factors that will impact the boiler size that will be most suitable.

Check out our boiler brand price guides and comparisons:

Boiler size calculator summary

Boiler sizing and marketing jargon can appear overwhelming when first looking into installing a new boiler, however, throughout this article, we have aimed to explain the considerations behind boiler sizing.

As discussed, there are various considerations to take into account when selecting the most suitable boiler type and the most appropriate boiler size and also, there are no benefits to over-estimating the boiler size needed.

Therefore, should you prefer more tailored advice, it is recommended that an appointment is booked with a heating engineer who would be able to provide expert advice on the best boiler options for the property.

Further reading: