How to Calculate Correct Central Air-Conditioning Unit Size

Air-conditioner units are ranked by the BTUs they produce.

Calculating the proper size for a central AC unit requires determining how much energy is required to pull heat out of the air. The calculation determines the BTUs (British Thermal Units), which can be easily converted to tonnage. Most central AC units are ranked by the tonnage of the units, such as 2.5 or 4.0 tons. The easiest method of determining the proper tonnage for your living area is to calculate the square footage of individual rooms and converting that to BTUs and tonnage.

Things You'll Need

• Measuring tape
• Calculator

Instructions

1. Measure each room individually for length and width including any hallways or storage areas that will be impacted by the central AC unit. Do not include rooms that are not connected or will not be impacted by the unit, such as closets, basements or attics.

2. Multiply the length of each room by the width to determine the square footage.

3. Convert the square footage of each room to BTUs. 100 to 150 square feet equals 5,000 BTUs. 150 to 250 square feet equals 6,000 BTUs. 250 to 350 square feet converts to 7,000 BTUs. Add 4,000 BTUs to kitchens and 1,000 BTUs to laundry rooms or bathrooms.

4. Add each room's BTU calculation together to determine the whole house BTUs.

5. Divide the whole house BTUs by 12,000 to determine the tonnage. Round the tonnage up to the nearest 5/10ths. For example, 4.1 tons should be rounded to 4.5 tons. 3.8 tons should be rounded to 4 tons.

Tips & Warnings

• Choosing the right unit means choosing one ranked for the rounded tonnage. Choosing a 7.0-ton unit for a house that was calculated at 4.0 tons means that humidity will not be able to be pulled from the air properly and the electrical consumption will be higher than necessary.

References

• Consumer Energy Center: Window Air Conditioners
• Alpine Home Air: Selecting Air Conditioner Size

Resources

• Read this Article in Spanish