How to Calculate BTU Per Square Foot

The British Thermal Unit (BTU) is the basic measure of heat energy in the Imperial system. One BTU is defined as the amount of heat necessary to raise 1 pound (0.454 kg) of water 1 degree Fahrenheit (0.55 degrees Celsius). Knowing the number of BTUs required per square foot of space is important in choosing the right size furnace and air conditioning system to heat and cool your house efficiently and effectively. You'll have to determine the amount of space to be heated or cooled and choose the right furnace or air conditioner unit for the job.

Basic way to Calculate BTU

Determine the square footage of each room to be cooled. 

Most rooms are either rectangular or triangular in shape. Rooms that aren't either of these shapes can be broken down into spaces composed of these shapes. An octagonal room can be broken down into a room with a rectangular center area and four ends each composed of a small rectangle flanked by a right triangle on either side. You would figure the square footage for each component area and then add them together to get the total square footage of the room.

· For a rectangular room or area, multiply the length by the width. (Remember that a square is a rectangle whose length and width are the same distance.)

· For a triangular room or space, multiply the length by the width and divide by 2 (the geometric formula of 1/2 the base times the height).

 

If the room is circular, multiply the distance from the center to a wall by itself and then by 3.14 (pi rounded to 2 decimal places). In the case of an octagonal or hexagonal room, you can also approximate the square footage by treating it as a circular room and measuring from the center to the edge of one of the walls.

One method for determining capacity is with the index below, as derived from the Energy Star website at

http://www.energystar.gov/index.cfm?c=roomac.pr_properly_sized .