How to find out how many BTUs it takes to heat a room

Updated July 20, 2017

BTU (British Thermal Unit) is a measurement of the energy used in heating and air conditioning. Knowing the BTUs a unit can produce is key when heating and cooling a room. Buying radiators or room heaters that are the wrong size results in inefficiency, wasted energy and over or under heated rooms. Equally important when calculating room heating is the ability to work out the number of BTUs needed for the size of the room.

Measure the room. You will need to know the room's area in square metres as well as the height of the ceiling before you begin determining the BTU required for room heaters.

Find the room's volume in cubic metres. Multiply the room's square metre size by the height of the ceiling. The total is the room's volume in cubic metres.

Rate the room's insulation. Use numbers 2, 3 or 4. Very good insulation is rated 2, average is 3, and 4 represents poor insulation.

Multiply the room's insulation rating by the room's volume. This will give you the total number of BTUs it takes to heat a room.


You can easily find a BTU calculator online (see Resources below).


Some heaters are measured in watts. To convert a heater measured in watts to BTU, multiply the number of watts by 3.4 to find the total number of BTUs.

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