The ‘heat output’ or ‘BTU’ calculator is a good way to work out how much radiator power you need in your home. There are several factors that can affect this. Our BTU calculator will help you work out the heat requirements for your rooms.

If BTUs are a measurement of how much energy is required in order to heat your room, then 1 BTU is the amount of energy to raise the temperature of 1 pound (0.45kg) of water by 1℉ (0.55℃).

Follow the guide below to help you calculate an approximate heat output required for your room. There are 3 steps to working out the BTUs required.

Step 1: Calculate the volume (m3) of the room _ Length x Width x Height

Step 2: Calculate the required radiator output.

For BTU/hr: Room volume (m3) x 153 


For Watts: Multiply BTU/hr by 0.293

Step 3:

Other factors should be taken into consideration to get the true BTUs required. Calculate these factors by adding or subtracting the factors that apply to your room:

 For a solid floor - 10%                              

 For an uninsulated cavity wall + 10%

 For a foam filled cavity wall - 20%  

 For no loft insulation +15%

 For double glazing - 5%                           

 For 2 outside walls +15%

 For upstairs bedrooms - 25%

 For northern facing walls +10%

 For 3 outside walls + 40%

 For high ceilings (3m+) +20%

living room


* This should be taken as a guide, if you require a specific heat output for your room we would suggest you consult a plumber or heating engineer to provide you with an exact BTU measurement.