How to Pick the Right Size for an Air-Conditioner

Document the width and length of the room using measuring tape. Multiply these numbers together to find the square footage of the room.

Compare the room's square footage to the capacity needed on a British Thermal Unit chart. Air conditioner strength is measured in BTUs put out per hour -- the chart indicates how many BTUs you need for your room. For example, if your room is 370 square feet, you need an air conditioner that puts out 9,000 BTUs per hour.

Adjust BTU capacity based on the amount of sun your room receives. Reduce capacity by 10 percent for shaded rooms and increase it by 10 percent for sunny rooms. A 1,500 sq. ft. room typically needs a 30,000 BTU air conditioner. If the room is heavily shaded, however, you only need 27,000 BTU.

Determine how many people are usually in the room and add 600 BTUs for every person that typically occupies the space. A two-person bedroom of 150 sq. ft. requires a 5,000 BTU air conditioner. A four-person bedroom of the same size, however, requires a 6,200 BTU unit.

Add 4,000 BTUs if your unit is used in a kitchen. Due to the higher temperatures caused by cooking, kitchens require more powerful air conditioners.