Calculate the size of your home in square feet. Find the area of each room by multiplying the length times the width. Note the number and types of rooms in your home.
Determine the base amount of Btu your central AC will need by using the AC sizing chart (see Resource). For example, if you have 1,050 square feet and an entire house to cool, you would start with a unit with a capacity between 19,500 and 21,000 Btu.
Determine whether your home is insulated. Each room not insulated or not under an insulated roof will add 4,000 Btu to the AC unit capacity.
Determine the number of heavily shaded rooms and the rooms that receive significant sunlight. Multiply the number of sunny rooms by 1,500 Btu and the number of shady rooms by -1,000 Btu.
For each room used as a kitchen, add 4,000 Btu.
Add the values found in steps two through five to calculate the total capacity required of your central AC unit. For example, if you had 1,500 square feet, one kitchen, two uninsulated rooms, two sunny rooms and three shady rooms, you would start with an AC unit capacity of 24,000 Btu, add 4,000 for the kitchen, add 8,000 for the two uninsulated rooms, add 3,000 for the two sunny rooms and subtract 3,000 for the three shady rooms, for a total of 36,000 Btu.
- Your central air conditioner will cycle on and off as the temperature of your home changes. When the temperature reaches the target temperature, it will turn off. If you purchase an AC unit that is too large, it will cool too quickly without decreasing the humidity level of your home.