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Boiler size calculator – What size boiler do I need?

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 properties 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).

The type of boiler required

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, as follows:

  • Combination boilers – Also known as combi-boilers, 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.

  • Heat-only boilers – This type of boiler only provides heat energy, 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.
  • System boilers – A system boiler is a closed system requiring a boiler and a cylinder system. System boilers are often most suitable for properties with low water pressure; however, most system boilers also require more space, than more compact boilers such as a combi-boiler.

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

Further reading:

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.

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 illustrate a larger size property which would require a higher output boiler.

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.

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.

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

Related reading: 

Boiler Size Calculator

Taking all of the above information into account, our 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.

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.

Recommended reading: 

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: