Boiler Size Calculator and What size kilowatt (kW) boiler do I need?

Written by James Elston

Updated on 4th December 2023

Posted on 3rd December 2023

Topic: Boiler Advice

How to pick the right size boiler for your home?

Let’s look at what factors do you need to consider to find the right boiler before deciding on a boiler size or boiler heat that your home needs.

Here’s what you need to know to answer the question – What Kilowatt boiler do I need?

The questions below will work as your boiler size calculator:

• The number of bathrooms

• The number of radiators

• The number of people in the home – the demand for hot water

Please note that the above information gives you a pretty good idea of ​​which boiler size and hot water cylinder you will need.

Ensuring that you get the right boiler for your home if you are upgrading to an A-rated boiler from an old inefficient boiler can save you up to £800 per year on your energy bills.

Boiler sizing: What kW boiler is best for my home?

By the way, people often refer to the “size of the boiler”. What this really means is its higher output, which is measured in kilowatts, or kW.

What is a Kilowatt? A Kilowatt or kW is a unit that is used to measure boiler outputs or flow rates. In simple terms; how much water can be used at once.

Therefore you need to ensure you get the best kW boiler for your home to ensure that your home is heated effectively and efficiently. The average home will need a 30 kW combi boiler.

Use our online fixed price tool to know the boiler size that your home needs.

You’ll see fixed prices for boiler replacements that are the perfect match for your home’s power needs. If you are looking for a new boiler get a quote now online in 20 seconds.

Boiler size calculator

Choosing a suitable size boiler is extremely crucial for two main reasons:

1. It has to meet the heating and hot water requirements of your home.
2. Opting for a larger kW than you need will be a waste of money.

To work out what size boiler you need for your home, use our boiler calculator here: Free Calculator

What size boiler do I need?

An under-sized boiler has difficulty in meeting your home’s central heating needs, resulting in a not powerful enough heating system for your radiators, hot water for your baths/showers or daily needs.

Opting for a larger size boiler for your house than needed will cost more money than is necessary and increase the amount of energy used by your boiler.

Choosing the right size/kW type of boiler will ensure that your home is warm enough and you always have enough hot water when you need it.

By not using the boiler calculator and guessing what size boiler you need and going larger will mean you have spent considerably more money on a boiler model that you do not need.

If the boiler you choose is not upscale, you may be wasting a lot of money on burning unnecessary gas.

Please Note: If you are planning an extension of your home that would include additional showers, It may be worth considering a boiler with higher power (kW) and a larger hot water tank.

These boilers cost more, but you will definitely need to invest as an underpowered boiler will not work properly and cost you more in the long run in gas and electricity bills.

Combi boiler size calculator for your home, based on kilowatt ratings

The size of the combi boiler for your home depends on the amount of bathrooms. Check the table below.

The boiler sizing calculator suggests if you have more than 1 bathroom in your home, you need a boiler with more power to handle the higher hot water demand, especially when people tend to shower at the same time.

What size combi boiler do I need? The more bathrooms you need heated, the larger the boiler you will need.

The combi boiler size calculator is listed below:

No. of Bathrooms or Showers	Boiler Size (minimum)	Recommended Boiler
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

Combi boiler size by the number of radiators

The amount radiators in your home will also help you decide what size boiler to get in your home. here is our quick boiler sizing guide for the number of radiators. Luckily there are many makes and models available in varying outputs available too, so hopefully, you can work out which output you need and get the right boiler at the right price.

• 1-10 radiators: 21-28kw combi boiler
• 11-15 radiators: 29-35kw combi boiler
• 16-21 radiators: 36kw+ combi boilers

Calculate combi boiler size based on the number of bedrooms in your home

You should calculate the size of the boiler based on the number of bathrooms/showers (as shown above) as these are the main reasons to increase or decrease the output.

The following is a rough guideline for the asked questions on combi boiler size:

What size boiler do I need for a 2 bed home?

What size boiler do I need for a 3 bed home?

What size boiler do I need for a 4 bed home?

What size boiler do I need for a large 5+ bed home?

	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

Boiler sizes for large homes with multiple bathrooms

Viessmann has managed to lead us into the future and to build a hybrid boiler – the Viessmann 111-W.

They merged the DNA of a traditional boiler and a modern boiler to create their own.

It is a energy-efficient combi boiler that stores the water within the unit enabling it to suit homes with up to 5 bathrooms without the need for an external hot water cylinder.

At Boiler Central we offer Viessmann boilers with a massive 12-year warranty.

How to calculate what size system boiler or regular boiler you need

Regular boilers are also called heat-only boilers, featuring a boiler, hot water tank and a cold water tank.

The hot water tank is usually found in an airing cupboard and the cold water tank resides in the loft in most cases.

The number of radiators are the main factor with regular boilers as well as the number of bathrooms that you have. Both of these give us an idea of the heat only boiler size that you will require.

If you are asking yourself what size boiler do you need for a certain amount of radiators, see the table below which gives you an idea of regular boilers for different radiator configurations.

What size boiler for…	Boiler Size Calculation	Minimum kW Size
6 Radiators	6x 1.5kW + 3kW	12kW
8 Radiators	8x 1.5kW + 3kW	15kW
12 Radiators	12x 1.5kW + 3kW	21kW
15 Radiators	15x 1.5kW + 3kW	26kW
20 Radiators	20x 1.5kW + 3kW	33kW

Converting from a conventional to a combi boiler size calculator

Believe it or not, the best combi boilers are great, cost-effective to operate and deliver instant hot water without delay from your shower, bath or faucet.

No wonder the conversion to a combi boiler is the most common New Year’s resolution in the UK.

Although converting can be a pretty expensive job, our Boiler Central will offer you prices for boiler upgrades that are pocket friendly.

See the table below to see what size boiler you need for the following amount of baths/showers:

Number of Baths/Showers	Boiler Size (Minimum)	Recommended Model
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

It is worth noting that the size of the combi boiler that you would need if you are changing from a regular boiler is no different than if you already had a combi boiler.

All you need to do is to consider your hot water demand as indicated above.

If you want to buy gas boiler replacement and new boilers, but have no idea what size you need, the click the buttons below to get a fixed price online.

FAQs

What is the best boiler for a small house?

The best boiler for a small house depends on what is perceived as ‘small’. You would generally say that a home with 1-2 bedrooms is classed a small house. This includes flats and apartments too.

Therefore based on a 1-2 bedroom small house, the best boiler we would recommend would be the Viessmann Vitodens 050-W 30kW combi boiler.

What size boiler do I need for a 3 bedroom house?

The best size boiler for a 3 bedroom house is again the Viessmann Vitodens 050-W combi boiler.

This is probably joint with the Worcester Bosch Greenstar 2000 which is the new boiler from Worcester in 2021.

Any kW rating from 25kW to 35Kw would be fine, however somewhere around 29-30kW would be most suited to a home of 3 bedrooms.

Get a fixed online quote on those two boilers in 20 seconds – click here

What size boiler do I need for a 4 bedroom house?

The best size boiler for a 4 bedroom house is the Worcester Bosch 8000 Style combi boiler OR system boiler, depending on your home type.

For another option, the Viessmann Vitodens 100-W combi or system boiler would be a great fit for a home with 4 bedrooms.

Either of the above in a 35kW rating would be perfect for a 4 bedroom home.

What size boiler do I need for a 5 bedroom house?

The best size boiler for a 5 bedroom house would be the Worcester Bosch 8000 Style in 35kW or 40kW size.

Usually in a house with 5 bedrooms there is a system boiler that heats the home. It would be best to keep the system boiler in a 5 bedroom house as there will always be hot water ready sorted in the tank.

A combi boiler of 40kW would probably be ok, but a system boiler would be much more practical and you would have no issues with hot water demand.

What happens if my boiler is too small?

If your boiler is too small for your home then you may find that you are requiring hot water but it is not running as hot as it can, or should be.

This will be down to having a boiler that is too small for the size house. You would be best to complete our online survey and see the boilers that we offer that would be perfect for your size home.

If you have moved into a property and think your boiler is too small, you should get a quote on a new boiler in 20 seconds – click here

How many radiators can my boiler run?

Many of our customers ask us how many radiators their boiler size will run. This is dependent on a few factors but generally speaking, the larger your home, the more radiators that you will have.

This means that it’s easier to categorise your boiler size requirements into how many bedrooms your home has, as the FAQ’s above show.

Although the above is not always the technical answer, below you can see how we work out what size boiler you can have for the amount of radiators:

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

How many radiators will a 30kW boiler run?

A 30kW boiler will typically run 15 radiators with ease. The boilers that we recommend for this range would be the Viessmann Vitodens 050-W combi boiler. Get an online quote on this boiler by clicking here.

How many radiators can a 24kW boiler run?

A 24kW boiler will run around 12 radiators in a typical home. The Vokera EasiHeat 25kW combi boiler would be an ideal choice. Get an online quote on this boiler by clicking here.

How many radiators can a 35kW boiler run?

A 35kW boiler will run 20-25 radiators with no problems at all. The Worcester Bosch 8000 Style boiler would be a perfect fit for this range.

Are hydrogen ready boiler sizes the same as gas boilers?

Yes. The output, power, and size calculations are all exactly the same. You just have the benefit of having a boiler that will avoid the gas boiler ban in 2025. Hydrogen-ready combi boilers (to 20% blend) are widely available from all the best manufacturers and can run on up to 20% hydrogen fuel straight out of the box.

This new technology is preparing for a potential switch over to hydrogen fuel blend instead of just natural gas as a way of lowering our greenhouse gasses and also attempting to keep our energy bills in the fuel crisis down.

As prices rise, alternatives to gas boilers will be sought and these are one of the most likely solutions moving forward. A full hydrogen-ready boiler that is currently being developed will be able to be changed with a few small parts to be fueled by 100% hydrogen in the future.

How to get the Best Price on the Right Size New Boiler?

Without bragging, Purchase from us!

We are a nationwide online boiler installation company with over 5 stars that comes from a dandy load of satisfied customers.

We are an online business hence our operating costs are low, which means that we transfer these savings to our customers to make sure they always get the best deal for a new boiler whether it is a conventional boiler, system boiler, or the best combi boiler.

Due to the volumes we have installed, we can also access the exclusive warranty of our partner manufacturers.

For example, we can offer a massive 12 years of protection for the Viessmann Vitodens 050 Series, while your normal installer will give you 7 years. 

If you need boiler size information or a quote, click here