BTU or British Thermal Unit – is the amount of heat needed to increase the temperature of 1lb (one pound) of water by one degree Fahrenheit. This is the imperial measurement, your Installer may work using metric values for this calculation; Watts. Either can be used to choose your radiator.

How does BTU affect radiators?

With radiators, the BTU measurement refers to how much energy is required to heat a room. The higher the BTU is, the greater the radiator’s heat output will be. How effective the radiator will be though, depends on factors such as the size of the room and how well insulated it is. A radiator’s ability to transfer heat will depend on its material, size and surface area as well as the water temperature within the system.

Delta T, or ∆T, specifically relates to the difference in temperature between the water circulating in the central heating system and that of the room or ambient temperature.

Flow in 90°C ,Flow out 70°C = Average water temperature inside radiator is 80°C.

If the room or ambient temperature is 20°C and the average water temperature inside the radiator is 80°C, the Delta T or ∆T value is calculated as 80°C - 20°C = 60°.

A Delta T correction factor allows end users and professionals to find out the actual output of a radiator or towel rail in the range of Delta T variations.

Eastbrook provides Delta T 60° outputs.

You can use the listed correction factors below to find the actual output at Delta T 50° and other Delta T's listed below:

∆T Correction Factors
60° 1
55° 0.901
50° 0.781
45° 0.699
40° 0.599
35° 0.513
30° 0.424
25° 0.338
20° 0.256
15° 0.179

Assuming a radiator or towel rail has a heat output of 750 Watts at ∆T (delta T) = 60°. At ∆T (Delta T) = 50°, the output would be 750 x 0.781 (from the table above) equating to 585.75 Watts.

Step 1. Room Details

Room Type

Step 2. Extra Details

What is below this room?
What is above this room?
Window Type
Type of Outside Walls
Number of Outside Walls

Please correct the above issues before calculating

BTU Results

Required BTU
Required Watts

You may need more than one radiator to meet the required BTU output.

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Our calculations are only estimated and are based on information you provide. Our BTU Calculator can only process the most common factors that affect heat loss and may not take into account all factors relating to your particular requirements. Any results provided by our BTU Calculator should not be considered 100% accurate and we accept no liability for any errors resulting from the estimates given.

For completely accurate BTU requirements, we recommend you to refer back to your heating engineer.

All towel rail/radiator outputs are based on Delta T 60°C (∆T60°C).

Please Note.

This is a guide only. A radiator with insufficient output may never raise the surface temperature of the radiator to the required level. Outdoor temperatures, external walls, insulation, heated adjacent rooms (i.e. above, below, and to the side) all have a contributing effect. The boiler should have sufficient capacity to supply all connected radiators to the sum of their maximum demand. Larger rooms such as living rooms may require more than one radiator, positioned evenly throughout the room. If this is the case, simply divide the required output between the number of radiators needed.