The Sizing Calculations for an Air Conditioner

Room Size

• Find the room size by measuring the width and length of the room. Multiply the width and length to find the square footage of the room. The square footage is used as a starting point to determine the size of the air conditioner needed.

Environmental Conditions

• After you have the square footage, add or subtract BTUs for shady rooms or sunny rooms. If the room is shaded most of the day, subtract 10 percent from the total BTUs needed for the square footage. If the room is on a sunny side of the house, add 10 percent to the total BTUs. For example, if you need a 10,000 BTU air conditioner for a room 400 to 450 square feet, add or subtract 10 percent accordingly (10,000 -- 10 percent or 10,000 + 10 percent).

Room Occupancy

• After you have the total BTUs needed for the room size and environmental conditions, you may need to add BTUs to the total if more than two people will occupy the room. Air conditioner BTUs are determined for two people in a room. For each person in addition to the two people in the room, add 600 BTUs. For example, if you will have four people in the room, add 1,200 to the total BTUs needed.

Type of Room

• The final sizing calculation is the room type. If the air conditioner will be in a kitchen, add 4,000 BTUs to the total needed. Kitchens have appliances that add heat to the room, which means the air conditioner, must be larger to cool the room with the added heat and humidity.

Energy Efficiency Ratio (EER)

• Besides the correct size of air conditioner, select a unit that has a higher EER rating. This energy efficient rating will mean the air conditioner costs less to operate because the units have high-efficiency heat transfer surfaces, fan motors and compressors. The EER rating is found on the yellow Energy Guide tag on the unit.