Sizing Radiators for a Room- Which Radiator to Buy?

When you are looking to select the best radiator for a room you will probably consider the wall space you have available and the style of radiator you like. But before you consider those two items we need to first determine what level of BTU and Watt output you need to heat your space (kitchen/bathroom/Living etc.) to ensure a comfortable living environment.


BTU Calculator | Heat Loss Calculator

Scroll to the bottom of the page for the use of our Free BTU Calculator for Radiators. This heat loss calculator should be used as a guide only. Always confirm with your plumber or architect before order.

Radiator Sizes

When we talk about radiator sizes it can mean two things 1. The dimensions of the radiator or 2. The output of the radiator i.e. how much heat in Watt or BTUs will it give. It does not naturally follow that the bigger the radiator the larger the output. This depends on the style and make of radiator. Therefore to select a radiator for your space, let’s determine what heat you need in the room for it to be comfortable. Then you can take a look at styles and sizes of radiator that give you that heat requirement.


Radiator Outputs—What size do I need?

Let’s determine the output (Watt or BTUs) that you require for your space. To get this figure we need to know the size of the room and what it is used for i.e. what would the average desired temperature be. Knowing the construction of the room (Walls, windows etc) lets us understand how quickly a room loses heat, then we know what level of heat we need to put into a room to maintain a desired temperature.


The Watt/BTUs of radiator (i.e Heat) you require is dependent on 3 main factors:
1. The size (volume) of the room
2. The temperature you wish the room to heat to e.g. 21ºC
3. How much heat the room loses (heat loss) through walls, windows etc. You are trying to calculate how much energy (Watts/BTU) you need to put into a room to reach a certain temperature given that X amount of this energy or heat will be lost through walls, windows etc.

A radiator is sized to provide the heat required for your room. This heat is usually referred to using the terms Watt or BTUs (British Thermal Units). 1 Watt = 3.412 BTU. E.g. I need 1000 watts/3412 BTU Use the BTU Calculator / Heat Loss Calculator at the bottom of this page to determine the outputs you need for each room.

Convert Watt to BTUs

How do I covert from Watt to BTUs? Multiply x 3.412
How do I covert BTU to Watt? Divide x 3.412

Understanding Radiator Outputs in Radiator Selection

Once you know the output that you need to heat your room e.g. 1000 Watt / 3413 BTU you can then start looking at radiators that supply this heat output. There is one final thing to be careful of and that is the DeltaT used by the supplier when listing radiator outputs.

A radiator’s output ability is calculated in part by considering the temperature that the water is when it enters the radiator. Also considered are the temperature of water returning to the boiler and the average room temperature.


Delta 60 (Δt 60ºC) is the British Rating System whilst the newer European Rating is Delta 50 (Δt 50ºC ). Many radiator shops still sell radiators by only mentioning the output of the radiator at Delta 60. But Delta 60 assumes the water entering the radiator is about 85⁰C, and todays condensing boilers supply water at lower temperatures.

So if you needed 1000 watt/3412 BTUs to heat your room, you are using a modern boiler, and you purchased a radiator giving an output of 1000 Watt/3412 BTU at Delta 60- you would not have enough heat for your room. The same radiator on your heating system would only give you about 790 Watt. See formula below for converting Delta T60 to Delta T50.


For this reason all of the radiator outputs listed on The Radiator Shop are only at Delta T 50. We do not agree with listing Delta 60 outputs as while they make the radiator look like more value for money, they will not give our clients the heat outputs they require or expect. If comparing radiators across suppliers please ensure you are using the same Delta T figure by using the simple converter below. 


Convert Delta T50 to Delta T60

  • To convert the Delta 50 output of a radiator to Delta 60 multiply Delta 50 heat output by 1.264
  • To convert Delta 60 output of a radiator to Delta 50 divide Delta 60 heat output by 1.264

The heat outputs of your chosen radiators are unlikely to be exactly the same as your heat requirement. So, always go for radiator size with a higher rather than lower heat output and fit thermostatic radiator valves to control the room temperature.

How do I calculate the Heat Loss of a Room?

To get a definitive calculation on this you need to take into account the area of all floors, walls, and windows. The U Values of all construction materials and other relevant information such as whether the building is on a sheltered or exposed site.

There is an older more general method which just multiplies the volume of the room by a factor of 40 to give you the number of watts required. There are similar calculations for BTUs. We recommend that you do not use such a general system. The old method of over-sizing your radiator “just to be sure” is an inefficient way of heating. Yes, you can use a thermostatic valve to turn it down, but you will be paying for a larger radiator that you don’t need to begin with.

The Heat Requirement Calculator or Heat Loss Calculator on this website was designed by an Irish energy expert and takes into account room dimensions, year of build, glazing area, type of glazing, type of room etc. It is an accurate system if the data is inputted correctly. In general, this calculator will return a much lower heat requirement for new builds than the traditional methods of calculation as mentioned above. This is mainly because our calculator takes into account the low U valves of materials in buildings constructed to 2008 building regs which are:
• Roof 0.20
• External Wall 0.27
• Floor 0.25

What Temperature do I need for my Room?

The following are recommended temperatures required for rooms depending on their type. When you use a BTU Calculator or Heat Loss Calculator it should always ask you what type of room you are looking to heat as a radiator will need more output to get a room up to 22ºC than 18ºC.

Kitchen/Hall/Toilet/Bedrooms 18ºC
Living Areas, playrooms, games rooms etc. 20-21ºC
Bathrooms (with shower) 22ºC