Green Central avatar

Written by

Last updated:

Boiler Size Calculator – Work Out The Right Boiler For Your Home

Boiler Size Calculator – What size boiler do I need?

So, you’re looking for a new boiler. But it’s all so confusing.

What exactly is a gas boiler? What do you mean, “size”? Why are there so many boiler types?

boiler size calculator

Then you found hope in a boiler size calculator. It was great and all…

Until they start asking you about complicated engineering terms. Until they start quoting a fixed price even before you identify what is it that you need.

Then you’re lost again.

Get a fixed price online in just 20 seconds – Click here

No worries! That’s why we’re here! And we can give you replacement boiler quotes on any boiler you like. Let’s find out which size boiler you need real quick if you are unsure.

Hot Deals Icon

The Best Boiler Size for You

Okay. To get straight to the point, the boiler size means the kilowatts (kW) needed. Not the physical size or different shapes.

Other than the cost of a new boiler which is a huge impact factor, choosing the best kW (or a.k.a the right size boiler) for you will depend on the following things:

Your House Type and Size

  • Number of bathrooms
  • The number of bathtubs and showers (take note: bathtubs and showers boiler size have different requirements.)

Number of Radiators You Have


Most households have 1 to 2 radiators in place. A general rule of thumb is to have 1 radiator for every 4 or 5m in the room.

Then again, the size of your radiator matters too. If it’s big enough, then perhaps 2 is just what you need.

Your Hot Water Demand at Home

  • Number of occupants
  • Heating frequency

You might know by now that choosing a new boiler or replacing an old gas boiler won’t be as easy as choosing new wallpaper.

But knowing the basics will make it less intimidating.

Here’s how to find the best boiler size for a great price.

Boiler size calculator – Quick guide

Here’s our quick guide of recommended boiler sizes for certain homes. This isn’t written in stone but you won’t go far wrong with these figures.

Boiler SizeBoiler TypeRadiatorsBathroomsOccupants
upt to 29kwCombi BoilerUp to 911-4
30-45kwCombi Boiler10-151-24-6
up to 20kwSystem Boilerup to 101-21-5
21kw – 40kwSystem Boiler11-201-44-8
up to 20kwRegular Boilerup to 101-21-5
21-40kwRegular Boiler11-201-44-8

Need something more specific? We’ve compiled a list of the manufacturers of their boiler models including their own guide and calculators too if you want to visit their respective websites:

  • Baxi Boiler Size Calculator
  • Ideal Boiler Size Calculator
  • Glow-worm Boiler Sizes Calculator
  • Vaillant Boiler Size Calculator/Guide
  • Viessmann Boiler Size Calculator/Guide
  • Worcester Boiler Calculator

If you’re already eyeing a brand, the official websites of these brands may also help you find a good fixed-price quote for your specific needs.

We also encourage you to research brand reviews to know the best deals and fixed prices for new models or repair rates before purchasing one.

Hot Deals Icon

Boiler sizing – The basics

boiler size basics

Before the size boiler, you’ll probably be asked for the type of boiler you are looking for first.

No need to scratch your head in confusion.

Here’s a quick rundown:

What size combi boiler does my home need?

Calculating the right size combi boiler for your home is really simple with tools like our free online boiler quotation tool here.

It basically asks you how many bathrooms and showers you have so that we can work out what boiler you need from a few calculations.

Below are a few benefits and risks of purchasing a combi boiler for your home:


  • It provides instant hot water and doesn’t have a water tank. It doesn’t store water, just straight up heating the water from the mains.
  • Easier and cheaper to install.
  • Requires less space.
  • It’s perfect for smaller homes.
  • It’s one of the more modern boilers.


  • Since it doesn’t store water, if it breaks down, you’ll have no hot water at all.
  • Not suitable for simultaneous use and strong water pressure. If you have higher heat demands, make sure to carefully find the appropriate size combi boiler.
  • Might not be compatible with traditional or old pipes.

If you need a little help choosing the best combi boiler then the below boilers are some beauties:

Viessmann 050-W Combi Boiler

Worcester Bosch 8000 Life and Style

Conventional Boilers

Also known as a “regular boiler”,heat-only boilers”, or the “traditional gas boiler”.

worcester 4000 combi boiler

The water comes from a cold water tank, goes through heating, then stored in a hot water tank ready for use.


  • Suitable for larger properties in need of more consistent heating and water pressure.
  • Best for bigger families or homes with multiple bathrooms.
  • Because of the hot water tank for storage, you’ll have reserved hot water even if your boiler malfunctions. You can enjoy hot water until the heat goes away.
  • Good as a backup heating source.


  • A conventional boiler is more expensive and more complicated to install because of more traditional and old-fashioned pipework.
  • It requires a larger space because of its many parts (though the modern gas boiler nowadays comes in a more compact sizing).
  • It’s not the most energy-saving boiler option.
  • If not situated in a well-insulating or well-heating area, the heat will easily dissipate.
Hot Deals Icon

System Boilers

A system boiler works like a conventional gas boiler, minus the cold water tank.

It heats and sources water directly from the mains, and stores them in a hot water cylinder.

You’re basically getting a good mix of the advantages from a heat-only boiler and a combi boiler. But if you’re a big fan of a combi boiler, this might not cut it for you.

Do a little self-assessment!

system boiler tank


  • Requires less space as compared to a conventional boiler.
  • Hot water storage will be a good backup for unforeseen malfunctions and colder days.
  • Like a combi boiler, they provide water and central heating but can handle higher hot water demand.
  • Because a system boiler sources from the mains, the water pressure will be stronger.


  • Must be stored in a well-insulated area to avoid losing the heat.
  • Larger than a combi boiler.
  • The heating is not as instant as a combi boiler.

You should have a slight idea by now about the boiler type fit for your space.

But to find a good size boiler for you, under these types are more factors to consider such as:

  • Boiler Brand
  • Boiler Model
  • Minimum kW Size
  • Boiler Efficiency

Phew. It’s getting more complicated, isn’t it?

No worries. The next sections are simplified for you.

Find Your Boiler Sizes and Models

Here are a few things to remember before we proceed:

  • The kW ranges pertain to the minimum kW size. Some boiler models work for different sizes as long as it meets the minimum kW requirement.
  • These are just general rules for the right size boiler.
  • It’s best to consult a Gas Safe registered engineer, of course. But it favours you greatly to have your own basic knowledge.
  • Basic rule: A bigger size boiler = higher water pressure = meets higher heat demand.

Combi Boiler

Combi boilers use more kW because they need more power for instant heating.

But how it works makes it more energy-efficient than a traditional gas boiler.

Because it also provides central heating, additional considerations such as how many bedrooms, how is the hot water demand, and additional showers can be very much helpful in finding the right size combi boiler.

24-27 kW Boiler

  • For a small-sized house/apartment
  • With 1 bathroom
  • With 10 radiators or less
  • Brand Model Recommendations:
  • Valliant EcoTEC Plus
  • Viessmann Vitodens 100-W
  • Ideal Vogue Max

28-34 kW Boiler

  • A medium-sized house
  • With 2 bathrooms or 1 family-sized bathroom
  • With 10-15 radiators
  • Brand Model Recommendations:
  • Worcester Bosch 4000
  • Viessmann Vitodens 050-w
  • Baxi 800
  • Worcester Bosch Greenstar 8000 Lifestyle
  • Valliant EcoTEC Plus
  • Viessmann Vitodens 200-W
  • Ideal Vogue Max

35-42 kW Boiler

  • A large-sized house
  • With 2-3 bathrooms; or en-suite bathrooms
  • With 15-20 radiators
  • Brand Model Recommendations:
  • Worcester Bosch 8000 35kw
  • Ideal Logic Plus Combi 35kW Gas Boiler
  • Vaillant EcoTEC Exclusive Green IQ 843 Combi Gas Boiler
Hot Deals Icon

Conventional / Regular Gas Boiler

Since regular gas boilers are “heat only boilers”, the radiators (rather than bedrooms, bathrooms, and such) are the more important considerations.

Basically, more radiators would need a higher power.

9-18 kW Boiler

  • With 10 radiators or less
  • Brand Model Samples
  • Ideal Logic Heat
  • Baxi 400 Heat
  • Glow-worm Energy Regular 18kW Gas Boiler
  • Potterton Promax SL Heat 18kW Regular Gas Boiler
baxi boiler

18-26 kW Boiler

  • With 10-15 radiators
  • Brand Model Recommendation:
  • Baxi EcoBlue Advance Heat 25 Regular Gas Boiler
  • Glow-worm Easicom3 25kW Regular Gas Boiler
  • Worcester Bosch Greenstar 24Ri Regular Gas Boiler

27-40 kW Boiler

  • With 15-20 radiators
  • Brand Model Recommendation:
  • Worcester Bosch Greenstar 8000 Life

System Boiler

Since system boilers more or less work like regular boilers, the radiator is also a good basis for deciding on the size system of this type.

9-18 kW Boiler

  • With 10 radiators or less
  • Brand Model Recommendations
  • Baxi 800 System
  • Ideal Logic Max System

18-26 kW Boiler

  • With 10-15 radiators
  • Brand Model Recommendations
  • Viessmann Vitodens 100-W
  • Baxi 800 System
  • Ideal Logic Max System

27-40 kW Boiler

  • With 15-20 radiators
  • Brand Model Recommendations:
  • Baxi Megaflo System 28 Gas Boiler
  • Worcester Bosch Greenstar 30i System Gas Boiler
  • Vaillant ecoTEC exclusive Green iQ 627 System Gas Boiler
  • Glow-worm Energy System 30kW Gas Boiler

Why Is the Right Size So Important for a Boiler?

We all know that finding a suitable size boiler is essential. That’s why you’re here.

But really, why?

One of the most asked questions is about undersizing or oversizing. Which is better?

Undersized Gas Boiler

  • An undersized gas boiler won’t meet your hot water requirements and heating demands.
  • If it won’t heat you up when you need it, might as well skip it and save on your bills.

Oversized Gas Boiler

energy efficiency
  • You may be warm and toasty, but you’re burning unnecessary gas.
  • They used to recommend going oversized as a backup for the colder days. But why waste so much when you have the option to find the right one?
  • Not only is oversizing bad for the environment, but burning extra gas also means burning extra money.

Do you see where we’re going? The answer is neither. Both have their pros and cons, and the best recommendation is to really find the right size.

But if we’re simply talking about which is better, consider these instead:

  • Oversize if:
  • In terms of keeping you cold on sudden cold nights and you want to have a sure backup.
  • Just remember this is over the expense of a higher bill and more wasted energy.
  • Undersize if:
  • You live in an area where it isn’t really that cold, and you just need a little heat from time to time.
  • You live alone and don’t really need heat 24/7.
Hot Deals Icon

Factors other than output to consider

We hope you’re having a clearer grasp or probably looking at specific models by now.

You are? Great!

But there are some more factors you might want to throw into the equation for better accuracy. And an extra knowledge won’t hurt!

Understanding Boiler Heat Loss

Other than the size of the home and the number of radiators, it’s also important to understand how fast heat dissipates in your space.

Especially with a system or conventional/regular gas boiler that stores water, a well-insulated place is needed.

Unlike a combi boiler that heats up the water right then and there, a system and conventional boiler will rely on the water tank.

Water Pressure and Boiler

Aside from the number of bathrooms, the bathtubs and showers boiler size also differs.

This has to do with the flow rate term you’ve probably been encountering.

water pressure and flow rate

Essentially, stronger water pressure will demand more power. This is a good consideration, especially for combi boilers.

Even if it ticks off the list above, expect a smaller size combi boiler to have slower and less water pressure.

Boiler Energy Efficiency

One of the concerns with a traditional gas boiler is that it requires and wastes a lot of energy.

Aside from shooting up your bills, the constant transferring from cold to hot water and the use of gas aren’t exactly environmentally friendly.

This is also why many are opting to slightly transition from a traditional regular boiler to the greener combi boiler, or completely switch from a gas boiler to modern boilers.

An example of a modern boiler is the Hybrid Boiler.

  • It works like a conventional boiler but with a renewable system in place.
  • It’s called a hybrid boiler because of its two energy sources: the air source heat pump and the ground source heat pump.
  • It switches to the most efficient energy source at that moment.

If you want to opt for greener options, choose a Condensing Boiler. The heat is recycled and reused, as opposed to a Non-Condensing Boiler that burns the gases using fuel.

If you noticed, the use of gas boilers – or anything gas actually – is not as supported nowadays because of its carbon footprint.

If you have the chance, you may want to read up on the best electric boilers here.

Changing From This Boiler to That Boiler

If you’re getting a totally new boiler, great. Just follow all the tips thus far and get a good size boiler quickly.

But if you’re coming from a particular boiler type and changing to a new boiler type, you don’t really have much choice unless you’re willing to go through big adjustments and new pipeworks.

What do we mean?

From a Regular Gas Boiler

viessmann vitodens 100-w
  • You can opt for another regular gas boiler: No problem. You’re just basically changing the unit.
  • You can completely swap that out with a combi boiler as long as your household demands allow: You’ll be saving a lot of money, and the environment will thank you.
  • You can switch to a system boiler: It basically has the same components, and you can save a few bucks on bills too.

From a Combi Boiler

  • Try your best to not go for a conventional gas boiler: More often than not, it’s not worth the hassle. The pipeworks are completely different, and the space adjustments will make you go crazy.
  • Get another combi boiler: If it’s really the most appropriate size boiler for your place, you can opt for another model or brand instead.
  • Go for a system boiler if you really need a higher-power boiler: It’s less bulky than a conventional gas boiler and can give you similar advantages to that of a regular one.

From a System Boiler

  • Likewise, try your best to not opt for regular boilers: Though their systems are similar, remember that it requires an extra tank. That means more space.
  • Go for a combi boiler if you realize you don’t need as much heat: You’ll save a few bucks, and you’ll get hot water more instantaneously!
  • Replace it with another similar boiler type.
Hot Deals Icon

Boiler Repair and Maintenance

Of course, don’t forget to include the repair costs in your decision-making process.

Don’t blow off everything into purchasing a new boiler, but end up struggling to maintaining it and then not being able to use your trusty gas boiler at all when you need it the most.


As mentioned, your manufacturers are likely to give a decent fixed price for repair and tests.

It helps, though, to initially assess the condition of your boiler on your own before asking for professional help.

Here are some things you can look out for:

  • Uneven heating:
  • If some rooms get more heat than others;
  • If some bathrooms have hot water and some don’t.
  • Unusual odour:
  • Though it’s called gas boilers, they shouldn’t smell like gas. If it produces an unpleasant odour, better have it checked as soon as possible.

How to get the right (and correct) size boiler for your home – Summary

gas safe register logo
  • The right size boiler is important. If you can, do not undersize or oversize.
  • Gas boiler size pertains to the kW, not physical dimensions.
  • It is best to consult with an expert or engineer about a preferred boiler type. Especially if you live in an apartment, there might be specific gas boiler types recommended.
  • It helps to learn more about the different factors involved such as gas boiler types, their fixed price from manufacturers, demands, capacity, and everything in between.
  • If possible, learn how to maintain your boilers before paying for an actual repair. The promise of a fixed price isn’t always the best option.

And It Boils Down to This…

The hunt for a new boiler is already taxing as it is.

Especially if taking into account the installation, but that’s why we have devised our online boiler comparison system.

In just a few clicks you can get prices on boilers perfect for your home. you can even spread the cost with boiler finance!

But trust us, it pays to have your own knowledge about the basics of a gas boiler before you make big decisions.

That, mixed with expert opinions, will make it so much easier for you to find the right boiler for a great price.

You still know your household best after all, and the best boiler should be the one that will suit you and your family’s needs in the most perfect way possible.

Hot Deals Icon
Green Central avatar

Written by

Read some of our superb solar panel installation reviews from Trustpilot

Recently had solar panels fitted by Green Central and I can’t speak highly enough of the team. The whole experience has been trouble free and I would recommend them to anyone thinking about having solar panels installed – 10/10!

Rob Schofield


Our installation is now complete! 8 solar panels fitted and the journey from initial consultation through to the end product has been seamless.
Hats off Green central, Credit where it’s due.
Will definitely spread the word!



Recently had six solar panels fitted on my house by Green Central and I couldn’t have asked for a better service. In particular, I have to highlight the exceptional communications through out, the over all smooth running process and completely stress free experience, all down to Green Central. I now have grid independence which is valuable beyond estimation.


Name *
Email *
Phone No. *
Post Code *
Name *
Email *
Phone No. *
Post Code *