The British Thermal Unit (BTU) is the unit for measuring the heating capacity of a heater. A BTU is the amount of heat needed to raise 1 pound of water temperature by 1 degree Fahrenheit. You can determine the number of BTUs needed to heat your space with just a few calculations. The BTUs needed to heat a room are determined by its volume, level of insulation and climate.
Measure the length, width and height of the room in feet.
Multiply the length times the width times the height to calculate the volume in cubic feet.
Multiply the room's cubic footage by 4 if its walls contain fewer than 3 inches of insulation. Multiply it by 3 if the insulation is at least 3 1/2 inches thick and by 2 if the insulation is 6 inches thick or more.
Double the figure you arrived at in Step 3 if you live in a cold climate or prefer indoor winter temperatures well above room temperature.
Convert watts to BTUs. Many space heaters list their capacity in watts. To translate watts to BTUs, multiply the listed watts by 3.4.