The number of British thermal units necessary in an air conditioner is determined in part by the square footage of the space to be cooled. Other factors include amount of sunlight regularly in the space, the type of space, and the number of people who regularly occupy the space.
Approximately 25 British thermal units are necessary to properly cool a space. To calculate square footage, rectangular or square spaces are measured by multiplying the length of the space by the width. The square footage of a triangular space is computed by multiplying the length of the space by the width, and dividing the result by two. If a space contains multiple shapes it can be broken into sections. The square footage of each section is added to calculate total square footage.
The overall calculation of necessary British thermal units based on square footage is reduced by 10 percent for a space that receives little sunlight, and increased by 10 percent for a very sunny space. Properly cooling a kitchen requires an air conditioner capable of producing 4,000 more British thermal units than that required in a similarly sized space elsewhere in the home. The standard calculation of necessary air conditioner capacity is based on two people regularly occupying a space. Each additional person requires adding 600 British thermal units to the standard calculation.