A BTU, short for British Thermal Unit, is a basic measure of thermal (heat) energy.
A BTU is defined as amount of heat required to raise the temperature of one 1 pound (0.454 kg) of liquid water by 1 °F (0.556 °C) at a constant pressure of one atmosphere. One BTU is approximately 1.054 to 1.060 kJ (kilojoules)

Available on iPad, iPhone and iPod.
This BTU Calculator can estimate the number of BTU's you will need to heat your desired room. It will also convert the calculations into watts. The App allows you to save the information so you can reference it at any time. Larger rooms such as Living Rooms may require more than one radiator positioned evenly throughout the room. If this is the case, divide the required output between the number of radiators needed

For a more accurate reading.
While every effort is made to give the closest calculation other factors may need to be considered. This calculator is for general use only and for a more indepth result we always recommend calling on the services of a qualified heating installation engineer.