How to Calculate the BTU Heat Load for a Room

A room's BTU, or British thermal unit, heat load is the amount of warmth gained over the course of a typical hour as a result of body heat, lighting fixtures and rays of the sun shining down on the ceiling, walls and windows. This data is important when purchasing a new air conditioner, as the closer an AC unit's BTU rating matches the room's heat load, the more efficiently it will run. Finding the heat load of a room in BTUs requires nothing more than a few measurements and some simple mathematics.

Tape measure
Calculator

Instructions

- 1
  Measure the width and length of the room and multiply the two numbers. Multiply the result by 31.25. This number is the BTU heat load for the area of the room.
- 2
  Measure the area of each window in the room. Multiply the area of all south-facing windows by 868, and those on other walls by 164. If a window has no curtains or blinds, multiply its result by 1.4. Add the numbers for each window together to find the BTU heat load for the room's windows.
- 3
  Multiply the average number of people using the room at a given time by 600 to find the BTU heat load for people.
- 4
  Determine the wattage of every light fixture in the room. This is usually printed on the light bulb. Add the wattage of all the fixtures together and multiply by 4.25 for the BTU heat load for lighting.
- 5
  Add the BTU ratings for the area, the windows, the people and the lighting together to find the room's total heat load in BTUs per hour.

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