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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

To determine what heat output (BTU or Watt) you need for each room, you can use our Free BTU Calculator for Radiators. This heat loss calculator should be used as a guide only. Always confirm with your plumber, builder, engineer or architect before order.

Radiator Sizes

When we talk about radiator sizes it can mean two things:

  • The dimensions of the radiator
  • 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 look at styles and sizes of radiator in our store that give you that heat requirement.

Radiator Outputs- What do you mean?

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 our Free BTU Calculator / Heat Loss 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


What Does Delta T Mean?

One important consideration when looking at radiator outputs is the DeltaT used by the supplier. DeltaT is based on the temperature difference between the water circulating throughout the central heating system and the ambient room temperature. The British rating system is Delta 60 (Δt 60ºC), while the newer European rating is Delta 50 (Δt 50ºC).

It's essential to ensure that you're looking at a DeltaT 50ºC output when shopping for radiators online. Many radiator shops still sell radiators by only mentioning the output of the radiator at Delta 60. But Delta 60 assumes that the water entering the radiator is around 85°C, and today's condensing boilers supply water at lower temperatures.

Suppose you require 1000 watt/3412 BTUs to heat your room, using a modern boiler, and you purchase a radiator giving an output of 1000 Watt/3412 BTU at Delta 60. In that case, you won't have enough heat for your room. The same radiator on your heating system would only give you about 790 Watt. It's crucial to convert Delta T60 to Delta T50 to ensure you're purchasing a radiator that will provide the necessary heat output.

All of the radiator outputs listed on The Radiator Shop are only at Delta T 50. We believe that listing Delta 60 outputs can be misleading as they won't provide the expected heat outputs. Suppose you're comparing radiators across suppliers. In that case, it's essential to use the same DeltaT figure and use the simple converter provided.

It's unlikely that the heat output of your chosen radiator will be precisely the same as your heat requirements. It's always advisable to go for a radiator size with a higher heat output than your requirements and fit thermostatic radiator valves to control the room temperature effectively.

Convert Delta T50 to Delta T60

  • If you come across a radiator with a Delta T 50 output but need to convert it to Delta T 60 for comparison purposes, you can use the following rough estimate:
  • To convert Delta T 50 output of a radiator to Delta T 60, multiply the Delta T 50 heat output by 1.264.
  • To convert Delta T 60 output of a radiator to Delta T 50, divide the Delta T 60 heat output by 1.264.


Radiator Outputs for Heat Pumps

If you are considering installing a low-temperature system such as a heat pump, it's essential to adjust the Delta T for your radiators. With lower water temperatures, you'll still want to maintain the same room temperatures, which means your Delta T will be lower than the standard Delta T 50. To determine the heat output of a radiator at a lower Delta T, you need to find out what the radiator will give out at Delta T 30 or Delta T 25.

At The Radiator Shop, we understand the importance of ensuring that our customers have access to the necessary information to make informed decisions about their heating system. If you need to know the output of a radiator on our site at these lower Delta T levels, please don't hesitate to send an email to Our team will be happy to assist you in selecting the appropriate radiator for your low-temperature system.


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


Choosing the right radiator for your home can be overwhelming, but understanding the necessary heat output for a room is crucial. Once you know what you need, you can browse radiator styles and sizes to find the perfect fit. Use our free BTU Calculator to determine the output required for each room, but always confirm with a professional before ordering. It's also essential to understand DeltaT, the rating system based on the difference between water temperature and room temperature, and make sure you're comparing radiators at DeltaT 50ºC. Remember to select a radiator with a higher heat output than your requirements and use thermostatic radiator valves to control the room temperature effectively.

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