The acronym BTU stands for "British Thermal Unit." The US standard counterpart is the joule, a unit of energy with a conversion factor of 1 BnTU to 1054.35 joules. Since the performance of a radiator is typically characterised by its power output or the rate of energy emission -- as opposed to total energy emissions -- BTU calculations for a radiator can determine the total energy requirement of the area to be heated by the radiator.

### Room Size

The volume of space to be heated is the main consideration when determining the total BTU output requirement of a radiator. As the size of the room increases, the BTU requirement increases in a direct proportion. The insulation of the room plays a role in this sort of calculation as well. In general, the better a space is insulated, the better it retains heat, and therefore the total BTU requirement decreases as the insulation quality increases.

### Area BTU Requirement

A good estimate of the BTU requirement "R" is the volume "V" of the space to be heated divided by an insulation factor "F." For average-quality insulation, this constant, F, will be around 500. More well-insulated spaces have higher insulation factors, whose value can increase to as much as 650. To find the requirement, simply multiply the length "l" by the width "w" by the height "h" of the room, and divide the result by the insulation factor: R = V/F = (l x w x h)/F.

### Heat Loss

No radiator is 100 per cent efficient at heating any space. A good estimate of the heating loss is around 20 per cent of the BTU requirement, calculated as the ratio of the volume of the space to its insulation factor. Therefore, the total BTU requirement for any space will be the volume of the space divided by the insulation factor plus 20 per cent of the volume of the space divided by the insulation factor: R = V/F + 0.2(V/F).