What size AC do I need? Rule-of-thumb tonnage calculator
The calculator gives a starting estimate of AC size in tons from your home's square footage and a few details. It is a rough guide. Square-footage rules ignore insulation, windows, sun exposure, ceiling height and climate. The proper method is a Manual J load calculation, which a contractor should run before quoting a replacement system.
How the calculator estimates size
AC capacity is measured in tons. One ton equals 12,000 BTU per hour of cooling. The calculator takes your conditioned square footage and returns the rule-of-thumb range in tons and BTU per hour. It asks for nothing else, because a rule of thumb cannot use anything else.
The US Department of Energy notes that a commonly used estimate is one ton of cooling for each 400 to 500 square feet of building area. It also says that estimate fails to account for local climate and the efficiency of the house, and that one ton can condition 800 to 1,000 square feet of a very efficient house. That gap is why the calculator shows a range.
Why square footage alone is rough
Two homes with the same floor area can need very different amounts of cooling. The load depends on:
- climate and how hot the design day is where you live
- insulation levels and air leakage
- the number, size, type and direction of the windows
- ceiling height and the number of floors
- duct location and duct leakage
- how many people live in the home
A rule of thumb cannot see any of these.
The proper method: Manual J
Manual J is the load calculation published by the Air Conditioning Contractors of America. The Department of Energy says your contractor should use it to calculate the proper size of the equipment. It works room by room from measurements of your actual house.
Bigger is not safer. The Department of Energy warns that oversized cooling systems can cost too much, provide inadequate comfort, and lead to mold and poor indoor air quality. An undersized system runs constantly and falls behind on the hottest days.
Do not accept a quote that simply matches the size of the old unit. The old unit may have been wrong from the start, and insulation or window upgrades change the load. Ask each bidder for the load calculation behind the size they quote. The guides to AC replacement cost and central air installation cost show how size affects price.
Common questions
How many tons of AC do I need per square foot?
There is no reliable per-square-foot figure. The Department of Energy cites a common estimate of one ton for each 400 to 500 square feet and says it ignores climate and house efficiency. A very efficient house can get 800 to 1,000 square feet per ton. Use the estimate to sanity-check quotes only.
How do I convert tons to BTU?
Multiply tons by 12,000. A ton of cooling is 12,000 BTU per hour, so a three-ton unit is rated at 36,000 BTU per hour. Residential equipment is sold in half-ton steps, and the size is usually coded into the model number on the outdoor unit's data plate.
Is it better to oversize an AC to be safe?
No. An oversized unit cools the air quickly and shuts off before it has removed much humidity, so the house feels clammy. The Department of Energy says oversized systems can cost too much, give inadequate comfort and lead to mold and poor indoor air quality.
Can I do a Manual J calculation myself?
You can gather the inputs, such as room dimensions, window sizes and insulation levels. The calculation itself takes software and training to do properly. For a replacement, ask the contractor to run it and give you a copy with the quote.
Related guides
Next step
Ready to talk to a pro?
We are an independent guide for homeowners, not a contractor. When you ask, we work to connect you with one licensed company that serves your ZIP code.
HeatCoolVerdict is not a contractor. We connect you with licensed local companies, and they pay us when you call or request a quote. It does not change what you pay. How we make money
Compare quotes from licensed local pros
Three short steps. No obligation to hire.