What Size Radiator Do I Need?

If you are undecided on what radiator size you need, read our list of factors for confirmation.
Written By: Richard Morgan | Updated:
What Size Radiator Do I Need
Richard Morgan/DIY Works

Whether you are replacing a radiator or having a brand new one plumbed into your central heating system, there are plenty of options to choose from.

From radiators to heat up your conservatory to chunky double-panel radiators to heat large rooms, you’ll need to decide on a size to determine the radiator heat output you need.

To help you decide on a radiator size, below are some factors you should consider.

Heat Output

When searching for your next radiator, you should pay particular attention to the BTU rating. In a nutshell, BTU stands for British Thermal Unit, which is a traditional unit of heat and the higher it is, the more heat the radiator will output.

Due to the fact that all radiators state this unit of heat, it allows you to calculate the amount of radiator power you require. B&Q have a great calculator that allows you to input the size of your room as well as other factors to determine the amount of BTU required to heat a room adequately. It’s important to note that there is no need to just have a single radiator in your room because adding multiple radiators will add to the overall BTU required.

Single vs Double Panel Radiators

While trying to find radiators with high BTU ratings, you may come across some single or double-panelled ones. In short, double-panel radiators feature additional fins inside the radiator, increasing its surface area and emitting far more heat than a single-panel radiator. Although more expensive, they are a worthwhile investment if you only have space for one radiator in the room. Below is an example of a large (1800 x 600 mm) double-panel radiator that we installed as a replacement to a single and it certainly made a huge difference.

If you have limited space for a radiator, a single-panel radiator will need to be much larger than a double-panel radiator. This is worth noting because you can install a smaller double-panel radiator with the same BTU output.

what size of radiator do i need

Richard Morgan/DIY Works

Location of the Radiator

Depending upon the location of the radiator in your home will determine whether or not you need additional heat. For example, hallways or large open-plan kitchens will need extra heat when compared to bedrooms. Although obvious, some people may overlook this factor and regret it during the colder winter months.

Style of Radiator

Whether you install standard or designer radiators, the BTU rating varies between the different styles. Although designer radiators look great, most don’t output the same BTU as a similar-sized standard radiator. Therefore, if you are installing a designer-style radiator, you may need to replace the radiator with a larger unit to meet the required BTU.


If you have a keen eye for aesthetics, it’s a reason why you may choose a specific radiator size in certain rooms. For example, in my own house, I wanted a vertical radiator to meet the top of the door architrave and leave a small distance in the middle (as shown in the image below). Although this involved tailored pipework, it was worth the effort of finding the radiator size I needed before purchasing the radiator.

what size radiator

Richard Morgan/DIY Works


To conclude, the size of the radiator you require mostly depends upon the size of the room. There is nothing worse than a cold room, and it’s important that you take your time to find the best radiator size for the room. Bigger is always better if you have the space in the room, and even if you don’t want it hot all the time, you can invest in some thermostatic radiator valves to keep the temperature comfortable.

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