What Size Boiler Do I Need for My Home (Best Online Calculators)

With hundreds of different boilers on the market and each offering different specifications, how do you know if you are choosing the right-sized boiler for your property? In this short guide, we will offer some expert advice on how to ensure you are picking the right sized boiler for your property.

Unless you have some prior knowledge about heating and boilers, choosing the right boiler can seem like an impossible task. Even if you know which make you want and have a range you like, the next decision is how to choose the right sized boiler.

  • Choose one that’s too big and you will be spending more money than you need.
  • Choose one that’s too small and you could end up with all sorts of problems like your water not heating up and radiators not getting up to temperature.

In this guide, we will outline some simple advice on how to choose the right sized boiler for your property. We will share the advice of our expert heating engineers, and we will recommend some excellent online tools to help you choose the right boiler for you.

Consulting a heating engineer

If you need to buy a new boiler for your property, the most common thing most people do is call out a heating engineer to tell them what size and type of boiler they need.

This is a fairly safe option although you will most likely be charged for the call-out and a lot of heating engineers have certain makes and models they recommend either because they get good deals on buying them or are familiar with installing them.

If you want a totally independent steer on the size of boiler to get, there are some other options.

Firstly, you can figure out the size of your previous boiler and, assuming that worked fine for your property, you can then find an equivalent model.

Alternatively, you can seek out an online tool.

Online tools to choose the right sized boiler?

If you consider buying your new boiler through an online installer, you will most likely be asked to fill in a simple form to help them identify the best boiler for your property.

This form will usually only take a couple of minutes to fill in and the site will then recommend a range of boilers that will work for your home. These boilers will all be of a suitable size for the property based on the information you have submitted, so from this list it is possible to deduce what sized boiler you need.

You can stop looking then, but it is worth giving some thought to buying your boiler from one of these online installers.

Sites like WARMZILLA will generate a fixed-price quote and in our experience, it is hard to beat what they have to offer on price.

They will list their recommended boilers with the cheapest and most suitable at the top. They will explain the price and also detail any special offers or extended warranty deals they have available.

WARMZILLA can help you to buy the right boiler for your property at a bargain-basement price and they can also help you save a bundle on the installation costs too. If you need to spread the payments, they have some terrific finance deals available too. Their standard APR is just 9.9% but they have regular 0% finance deals available too.

Other Boiler Size Calculators

WARMZILLA offers the best online boiler size calculator we have found. It can generate recommended boilers in just a couple of minutes and you don’t have to submit any private information.

However, they are not the only site to offer such a tool and some others you might want to look at include:

Understanding What Size Boiler Do I Need?

While online heating installation sites and some boiler manufacturers offer boiler size calculation tools as standard, not all do. So what size boiler do you need?

With most manufacturers, you will find that there are three main groups of combi boiler sizes:

  1. 24kW – 27kW
  2. 28kW – 34kW
  3. 35kW – 42kW

These three size groups can be identified easily using the following boiler sizing chart:

Boiler Sizing Chart

Property Radiators Bathrooms Boiler Size
Small Apartment / 1-2 bed house. Less than 10 1 24kW-27kW
Large Apartment / Medium-sized 2-4 bed house 10-15 2 28kW-34kW
Larger Properties 15+ 2+ 35kW-42kW

It is important to stress that this boiler sizing chart is an estimation and there are a number of important caveats you need to bear in mind when using it:

  • Reasonable hot water demand – The table assumes that you use an average amount of hot water. If your hot water usage is higher than usual, you might need a bigger boiler to cope with it.
  • Average-sized radiators – It also assumes your property is fitted with average-sized radiators. If you have larger radiators, you may again need to opt for a bigger boiler.
  • Average usage – Combi-boilers prioritise hot water, so if you are using hot water, it will direct resources away from the heating to deal with this demand. This is fine as long as your usage of both hot water and heating is average. If you regularly crank up the heating to the max and then want to take a long, hot shower too, a more powerful boiler might be more suitable for you.

Boiler recommendations by size

Now that we have explained what sized boiler you should be thinking about, the next question is what model is the right one for you.

This will depend on a lot of different factors such as your budget, preferred manufacturer, and personal preference.

But we set our panel of expert heating engineers the challenge of identifying the best boilers they know of different-sized properties. To keep the list manageable, we asked them to restrict their answers to our three top boiler manufacturers; Worcester Bosch, Viessmann and Ideal Boilers.

Here is a summary of their responses:

Small 1-2 bed property with up to 10 radiators

  1. Worcester Bosch 25SI Compact
  2. Viessmann Vitodens 050
  3. Ideal Logic Plus 24

Medium-sized 3-bedroom property with up to 10 radiators

  1. Worcester Bosch Greenstar 25i or 30i
  2. Viessmann Vitodens 050
  3. Ideal Logic Plus 30

Above average-sized 4-bedroom property with up to 14 radiators

  1. Viessmann Vitodens 100
  2. Worcester Bosch Greenstar 42CDI Classic
  3. Ideal Logic Plus 35

Large property with 5 or more bedrooms and more than 16 radiators

Our experts unanimously stated that if your home is this size or larger, it was essential to get a heating engineer out to make all the necessary calculations and ensure you get the right boiler.

An online boiler calculator such as the one offered by WARMZILLA will offer some guidance but in a property of this size, it is better to get an expert on-site opinion.

This is a recommendation we are in full agreement with.


Choosing the right sized boiler is every bit as important as choosing the right make or model. If you opt for the wrong size, you will either waste money or end up with an inefficient heating system.

In this guide, we have outlined what size boiler is suitable for different sized homes and recommended some of the best models for different sized properties.

We have also shown you how to use online tools to work out what sized boiler you need, with WARMZILLA being the most effective tool as well as the easiest to use.

If you have any other questions about boiler size that we haven’t covered in this guide or any other tips for our readers, please do share them with us using the comment box below.

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