What Size Radiator do I Need? Best Size Radiators To Fit Your Home Guide

James Elston
Written by James Elston
Updated on 30th November 2023
Posted on 15th November 2023
Topic: Boiler Guides
What Size Radiator do I Need

What size radiators do I need in my home?

Whether you’re upgrading an old heater or wanting to install a new radiator, it’s important to choose one that is the right size for your room. This could be the difference between finding a radiator that is cost-effective or not. Nobody wants to pay more on their heating bills than necessary!


New, modern designs mean that your home can have radiators that effectively heat your house without compromising on the style.

You’re no longer restricted to the chunky white radiator design. Contemporary designs can even be incorporated into a room’s feature piece. From a column radiator to heated towel rails, you can pick the design specifically for your house’s needs.

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The shape and size of your radiator will likely dictate where you place it in your central heating system. You can mix and match different styles to optimise your space and allow for the best heat output. With so many varieties of radiators to choose from, selecting a new radiator has never been so easy.

What size radiator do I need?

When it is time to replace your radiators, you can use a BTU (British Thermal Unit) calculator to work out which size radiator your room needs. This measuring system is a quick and easy way to estimate how much heat a radiator will need to emit to warm your room.

The BTU calculator will take several factors into consideration when working out the heat output requirements. The output levels will be impacted by the room size, the number of windows, and the location of the room.

Radiator designs have modernised over the years and can be found in all shapes and sizes. From a column radiator to heated towel rails, you can choose the style that works for you.

Each type will emit varying heat levels, so you have to pick one that fits your room’s BTU requirements. Sometimes it might even be better to have multiple small radiators rather than a single large one.

It’s important that you install the right sized radiator for each room, otherwise, you might end up with overpriced energy bills or a cold house. It’s important to factor in the radiators when looking at your new boiler cost figures as they will need to be accounted for.

Continue reading for advice on what type of radiator to buy and the best place to install it. If your planning on getting new radiators, to get the best out of them it may be a good idea to give the central heating system a power flush to ensure that no debris or sludge will be building up moving forward.

Working out the BTU (British Thermal Units)

BTU stands for British Thermal Unit. This form of measurement is the standard way to calculate how much heat output your room requires and the radiator size you will need.

The BTU is commonly based on the cubic volume of space, which is the room dimensions. This is calculated by the height, length and width of the room multiplied by four. You can measure these using a tape measure. The calculations often vary depending on the additional heat requirements.

The type of window, such as double glazing or French windows and the location (north or south facing) will affect the BTU calculator result. A BTU calculator will save any confusion by automatically doing the calculations based on the measurements and factors you input. Alternatively, you can ask a plumber to do the measurements and calculations for you.

The basic factor that you need to remember is that the higher the BTU, the higher the heat output. You’ll need to find a radiator size that gives your room the required BTU output it needs. An accurate calculation will save you money and energy.

If the radiator you choose has a BTU output that is too low for the room, you won’t get enough heat. If the BTU output is too high because the radiator is too big, you’ll end up paying more on unnecessary heating bills.

Your total BTU output doesn’t have to be calculated for a single radiator. The BTU requirement can be reached by combining multiple radiators to heat the room. This is helpful if your room is large and features an extension or has several cold spots.

Check out the best radiators to buy right now in the UK in terms of value and energy efficiency.

How much heat escapes from a room?

The level of heat escape varies from room to room. It usually occurs via outside walls, windows and the roof. If these areas aren’t properly insulated, the heat emitted from the radiator won’t do much to raise the temperature in the room. Your boiler will end up working harder than necessary. Even the best combi boiler will struggle to heat a home with poor insulation.

To combat heat loss, you should make sure that your rooms are properly insulated. Good insulation is key to preventing heat escape and lowering your energy bills. An estimated 25% of a house’s heat escapes via the roof. An insulated loft and roof can increase the heat containment and raise your home’s overall temperature.

Another way to keep heat inside the room is to build a shelf directly above the radiator. This will trap the heat as it rises and send it back out into the rest of the room.

Try not to cover your radiators with clothes. This can block the heat from getting out into the room, causing your heating system to overwork and wasting energy. It can also increase the humidity levels, which might cause mould.

How do I insulate a room?

It won’t matter that you have powerful radiators if your house is poorly insulated. Your radiators will end up working overtime only to have significant energy loss.

There are several ways that you can insulate the rooms in your house to reduce heat loss from your new boiler, the quickest and easiest method is to keep the doors shut. This will trap the heat inside the room and prevent drafts from blowing in and lowering the temperature.

You should also invest in some thicker curtains for the windows. One of the most common ways to lose heat is through windows, especially if they are only single glazed. Thick curtains don’t fully keep the cold out, but they will trap the cold air and stop it from fully entering the rest of the room.

As mentioned previously, you also need to insulate your loft and roof. There are two popular ways to insulate your loft. This includes loft floor rolls and insulated loft floorboards. Loft floor rolls are usually made of materials like fibreglass and mineral wool.

As the name suggests, the material can be rolled out across your loft floor. Insulated loft floorboards are designed to fit over loft floor rolls and are made of polystyrene insulation and a rigid chipboard top layer.

The area that lots of people forget to insulate is the front or back door. The chances are that you’re losing more hot air through the doors than you realise. Buy draft excluders for both the door and letterbox to seal any gaps and prevent cold air from blowing in.

Does the type of room affect the radiator size?

As you would expect, a larger room will require a radiator with a higher BTU output and a smaller room will have a lower BTU output. To put it simply, larger rooms usually need larger radiators, with the same principle applying to smaller rooms. Some rooms may need multiple radiators to reach the BTUs required.

However, the size of the room isn’t the only thing to consider. The type of room also calls for different temperatures. Always remember that you can control these temperatures better with a thermostatic radiator valve for more energy efficiency.

These can be varied depending on your personal preference and the temperature outside, but usually, the room temperatures are approximate as follows:

  • Living room – 20°C/68°F
  • Bathroom: 21°C/70°F
  • Kitchen: 20°C/68°F
  • Bedroom: 18°C/65°F

Your kitchen will contain lots of appliances that already give off heat, such as the oven and fridge. Your kitchen’s temperature will increase whilst you are cooking, or if you have larger windows with plenty of natural lighting. The constant temperature fluctuation should be taken into account when trying to calculate the BTU.

Bedrooms usually have the lowest BTU in the house, so the radiator can be smaller. Depending on which side of the house your bedrooms are on, the sunlight might offer a natural heating system too.

Don’t forget to factor in the number and size of the windows and each room’s current insulation and make sure that your boiler replacement will cope with the number of radiators you are looking to power.

Where should I position the radiator?

Traditionally, radiators were placed in the coldest part of the room. For most houses, this would be under the window, especially if you have single glazed windows.

Although windows are still a common source of heat escape, more and more people are placing their radiators based on the room’s aesthetic. You can choose specific radiator types to compensate for things such as lack of space or colour scheme.

The second coldest part of the room is usually the outside wall. Some people think that placing a radiator here is counterproductive because it will take longer to heat the rest of the room. However, this is personal preference and the layout of your room.

Sometimes it’s more efficient to have two smaller radiators positioned at different points in the room than one big one. This will spread out the hot air output, rather than the heat source only being at a single point.

Furniture tends to absorb heat, so try not to place a sofa or cabinet in front of the radiator. Curtains can also be used to keep cold air out. If the curtains cover a radiator, however, they will block hot air from reaching the rest of the room.

Despite these factors, it’s pretty much up to you to decide where you want the radiator to go. You may base your decision on practicality or how fast you want the room to heat up.

What type of radiator should I get?

Unlike years gone by, there is now a wide range of radiator types to choose from. You can mix and match radiators to fit each room’s size and style. Below is a breakdown of the benefits and layout of each radiator type.

Single panel

The single panel radiator is best suited to smaller rooms. The single long panel has a smaller surface area and consequently emits less heat. Single panels also take up less space as they don’t stick out from the wall very much. This is perfect for rooms that are particularly narrow.

Double panel

Double panel radiators, also known as Type 22 radiators, are made of two steel panels and one row of convector fins. This type of radiator has a larger surface area than single panels, which means that more heat is emitted. Double panel radiators are better suited to medium to large rooms.


Horizontal and vertical radiators have the same heat output. This means that the choice between the two rests solely on your personal preference.

Horizontal radiators are the traditional radiator style and so suit traditional builds better. They can be bulky and take up more wall space than other types of radiators. However, it’s easier to swap a Horizontal radiator with the same kind as you don’t have to move the pipework. This can save on both time and installation fees.


Vertical radiators are one of the most popular designs at the moment. The modern, contemporary look can act as a statement piece in a room.

The installation can take more effort as you will likely need to change the pipework. However, as the radiator is placed vertically against a wall, they take up significantly less space than a horizontal radiator.


This design has a retro vibe to it and is very much a statement piece. Column radiators are popular in larger rooms with high ceilings. They have a large surface area, which gives off more heat. They also take up a lot of space, so you will need to have a wide area to place one.

This type of radiator is usually made from two hollow tubes. They can vary in size depending on how many columns they have, so they are suitable for rooms of all sizes.

Heated towel rails

As the name suggests, this radiator is designed to heat your towels. Although stylish and multi-purpose, they aren’t usually powerful enough to heat a large bathroom. In this instance, your bathroom might require another radiator in addition.

Heated towel rails can be a stylish addition to a bathroom and suited to rooms low on space. Some of these radiators can even be used even when the central heating is turned off. Dual fuel towel rails can generate heat using both electricity and the central heating system. This will save you energy when you just want to heat up the single radiator.

Infrared panel heater

Infrared heating, a radiant heat source, does not heat the room. Instead, it emits an invisible energy wave that heats the surrounding air. Infrared heating panels can be very energy efficient and used along gas central heating systems.

Should I turn off radiators in rooms I don’t use?

It can get expensive trying to heat every room in your house. Depending on the size of your house, you might have multiple radiators dotted about. You should consider turning the radiators in there off if there are rooms that you don’t frequently use.

Boilers work harder to keep a constant temperature throughout all of the radiators. By turning off one or two radiators, the temperature won’t decrease as quickly. This means that the boiler can reach the optimal temperature faster. The boiler can then be turned off sooner and thus save you money on the energy bills.

Keep the windows and doors shut to keep the room insulated and reduce drafts. It’s worth turning the radiators on low and opening windows every now and then. This will ventilate the room and reduce the chance of damp. You could also place a dehumidifier in the room. North facing rooms are particularly at risk of mould and dampness.

When should I replace my radiator?

Whilst you might initially blame your boiler for poor heating, you should also look at your radiators. Radiators put up with years of use, but they still suffer wear and tear like any other appliance. The issues might not be noticeable until it’s too late. Ensuring you have the right type of boiler is also important and should be taken into consideration.

Although your current radiator might seem to be working well, older radiators could be costing you more money. Poorly heated houses can be costing you. Draining your central heating system could solve this issue too.

You should aim to update your radiator about once every 10 years. This can vary depending on the quality of your radiator, as some only need replacing every 15-20 years.

Alternatively, you might just want to upgrade your old, clunky radiator for something sleek and stylish. Modern radiators have improved efficiency than their predecessors and come in many shapes and sizes.

Bleeding the radiator

Before you start radiator shopping, it’s worth giving your radiator an inspection or asking a plumber to have a look. If the radiator doesn’t seem to be heating through completely, it might need bleeding.

Air can become trapped inside the radiator, which will stop the hot air from circulating. This can leave the radiator cold on top, but hot at the bottom. It’s also worth bleeding the radiator if it’s making noises, such as banging or gurgling.

To bleed the radiator, you will need a radiator key. Turn off the heating and then insert the key into the valve at the top of the radiator. Turn the key anticlockwise until you hear a hissing sound. This is the air escaping. Retighten the valve once the hissing sound stops. You may need to adjust the boiler pressure after bleeding the radiator.

We hope you found this guide helpful for selecting new radiators for your house. Now you know more about the different sizes of radiators available on the market you can feel more confident about purchasing your new radiators.

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Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 15th November 2023
Topic: Boiler Guides

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

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