What Size Radiator Do I Need?

Radiators are an important part of our homes, particularly when the dark nights begin to come in and the weather turns to the chillier side. And, getting the sizing right is essential if you are going to be able to suitable heat your home.

Thankfully, learning what size radiator a room requires is easier to get to the bottom of than you might think.

This article will teach you everything there is to know about what size radiator you need, what factors will influence the size, and how to measure for your new brand radiator properly.

Radiator sizing guide

Thankfully, discovering what size radiator you need isn't a complicated affair, but there are some considerations you need to make before making a purchase.

In order to get the perfect amount of heat into a room with a new radiator, there are four things you need to keep in mind. They are:

  • The BTU output required
  • How many radiators are required
  • How much available space do you have
  • And lastly, whether you would prefer steel or cast iron radiators

Don't worry; we'll explain each of these in more detail below.

So, let's get started.

BTU explained

The very first thing to do when deciding what size of radiator you need is to use a BTU calculator. But what exactly is BTU heat output?

BTU stands for British Thermal Unit and is simply a measurement of the energy required to increase the temperature in a particular room. When this measurement has been accurately calculated, you will have an answer to what radiator sizing you need.

It's highly recommended that you use our BTU calculator because working it out yourself can be a complex affair. It's also important that you know the precise cubic footage of the room you want to place a new radiator in.

When calculating the British Thermal Units required, a few other considerations need to be noted.

These are:

The temperature requirement of the room

Not all rooms require the same temperature. While bedrooms and living rooms tend to be kept warm and cosy, places like the hallway and kitchens are typically a little cooler since we spend less time in them.

The direction of the room

Believe it or not, the direction your room faces will affect how much heat it requires. For example, a room that faces north will usually be on the slightly chillier side and, therefore, will require more heat.

The types of windows you have

Window type

Double-glazed windows mean that a room will not require as much heat as double-glazing keeps much of the heat from a room inside, whereas a room that has French windows will mean there's a need for more heat in that particular room.

How BTU influences the size of radiators

Knowing the correct BTU output is vital so your home is properly and efficiently heated. If this calculation is incorrect or guesswork, it can cause many issues. You may find your hot is constantly too cold or uncomfortably warm, so remember to use a BTU calculator to avoid unnecessary hassle!

If you have more than one radiator in a room, once you have calculated the BTUs required, all you need to do Is divide this figure by the number of radiators in the room to give you the correct radiator size.

Measuring a radiator

Once you have your measurement for BTU, it's time to make sure that a radiator will fit into the space where you want it to go.

To correctly measure a radiator, don't measure the panel itself; measuring from each radiator valve at either end of the radiator is important. This means the pipework can be left as it is and won't require moving. It's also recommended that you choose a radiator that has a similar depth as it makes the installation process much more manageable.

That's not to say you can't make the leap to a smaller or larger radiator if you want to. Oftentimes, when a radiator has seen better days and is particularly old, or the house insulation was added later, a change in radiator size can be a good thing.

It will require a plumber who will reposition the pipework so that it needs the requirements of the new radiator size.

Can a radiator be too big for a room?

Many people think that getting the largest radiator that will fit a room and simply turning it down when it becomes too warm is a good way to take out the complexity of choosing the best size radiator for a room.

However, it's not cost-effective to buy a bigger radiator than you need. Why spend more money when you don't have to?

The correct radiator size should be within 10% on either side of the BTU measurement. However, if it comes down to buying a slightly smaller or slightly bigger radiator, then the best option is the latter.

Positioning a radiator

Radiators would traditionally be placed in the chilliest areas of a room and in many homes, which meant they were installed by windows, where more heat would inevitably escape. This is still true to an extent in homes where there is no double glazing.

Usually, a room's second most cold part is where the outside wall is. It's a common belief that a radiator shouldn't be placed in this area because it will take longer to heat the entire room, but this is down to the particular layout of the room, as well as a personal choice.

Nowadays, people are thinking about radiator placement more in terms of aesthetics. You can choose between a wide range of designs and colours to perfectly complement your space.

Sometimes is much more beneficial to have multiple smaller radiators in larger rooms rather than one bigger one. So, consider how much heat multiple radiators will bring if you have large room dimensions.

When deciding where to place your radiator, try to install it somewhere where furniture won't block it. This is because furniture can block and absorb heat, with curtains being the same. Curtains placed behind a radiator, however, are great as they will keep a lot of cool air out while not allowing warm air to escape.

However, where you place your radiator is completely up to you, but some of these factors would be helpful to keep in mind.

Radiator guide: choosing a radiator

There are many different types of radiators available, so we've discussed a few below to give you an idea of some of the designs out there today.

Cast iron or steel

A cast iron radiator tends to need more room than the usual steel types of radiator. As they weigh more, they are usually fixed to the floor instead of wall-mounted. However, certain kinds of cast iron radiators can be attached to a wall space.

Although heavier, these iron radiators can provide much more heat efficiently. They may take longer to heat up, but cast iron will retain this level of heat for much longer. This means the heat will continue to move throughout the room even after the heating has turned off. If you have a large room, these radiators are ideal.

Single or double panel

If you already have a single-panel radiator, you can opt for a double-panel radiator if you are happy with the size of the radiator requiring an extra bit of heat. A double-panel radiator will provide any room with a higher heat output due to its bigger surface area and are ideal if you have larger rooms. Meanwhile, a single-panel radiator is great for smaller rooms.

Horizontal radiators or vertical radiators

A vertical radiator is much more noticeable than a horizontal radiator and is found by many to be much more aesthetically pleasing. These types travel from the floor up to the wall and provide lots of heat directly into the room. Also, if you have a smaller room, vertical radiators are great for space-saving as they are slimmer.

Horizontal radiators are a more traditional option that can sit under windows or just about anywhere you want to put them. If you are replacing radiators of this kind, you might want to consider the same design as it will make installation easier.

Shop the range of radiators at PlumbNation


At PlumbNation, we have everything you could need to help keep your house feeling like a home. Why not check out our impressive range of radiators to keep you and your family warm and cosy this winter?

For more radiator advice, discover how to remove a radiator or how to fit and change a radiator.