Central AC tonnage calculator

How the calculator works

The DOE Energy Saver guide to home heating and cooling gives two figures. The common rule of thumb is one ton of cooling for each 400 to 500 square feet, which DOE says does not account for climate or the efficiency of the house. It also says one ton can condition 800 to 1,000 square feet of a very efficient house.

• Rule of thumb tons = floor area ÷ 500 to floor area ÷ 400.
• Efficient house tons = floor area ÷ 1,000 to floor area ÷ 800.
• BTU/h = tons x 12,000. One ton of cooling is 12,000 BTU per hour.

Worked example

A 1,500 square foot house: 1,500 ÷ 500 = 3 tons and 1,500 ÷ 400 = 3.75 tons by the rule of thumb. For a very efficient house, 1,500 ÷ 1,000 = 1.5 tons and 1,500 ÷ 800 = 1.9 tons. That spread, from 1.5 to 3.75 tons, is the reason square footage alone cannot pick a unit.

What to do with the result

Ask the contractor for a written ACCA Manual J load calculation, which accounts for your climate, insulation, windows, air leaks and ductwork. Our guide on what size central air unit you need explains what a Manual J should include and what to ask. If you are only cooling one room, the room air conditioner BTU calculator uses the ENERGY STAR chart instead.

Limits

This is arithmetic on two published rules of thumb. It is not a load calculation and should not be the only basis for buying equipment. An oversized system short-cycles and leaves humidity in the air.