AC tonnage calculator
This AC tonnage calculator turns the size of your house, your weather and the sun on your windows into tons of cooling.
Where the cooling comes from
Why a square feet rule misses
You may have heard "one ton for every 500 square feet." It is easy. It is also a guess. It treats every house the same. It does not see the sun on the west windows, the R-value of the attic or the heat lost in leaky ducts.
This page uses the same heat flow math as a full load calculation. That is why the answer moves when you change the ZIP code, the insulation or the sun.
Half tons and the 90 to 115 percent rule
Air conditioners come in half ton steps. A common first check is to pick a unit that covers 90 to 115 percent of the cooling load. A variable speed unit can go up to 130 percent. Below 90 percent the house may run warm on the hottest days. Far above 115 percent the unit short cycles and leaves the air damp.
The unit's real capacity changes with the outdoor and indoor air. The maker's data makes the final call.
The math in plain words
We add up the heat that enters through walls, roof, floor and glass, plus sun, air leaks, people and ducts. We do it hour by hour and keep the hottest hour. Moisture in the air adds more load, called latent load. The total is your cooling load.
Get the full version in the app
The web page gives a fast estimate. The iPhone app does the whole job.
- Every window, wall and duct, not just six choices
- Load split by room with CFM and duct sizes
- A PDF report with your logo
- Works with no signal. Your jobs stay on your phone.
Questions people ask
What size AC do I need for 1,500 square feet?
It depends on your weather, insulation, windows and ducts. Enter 1,500 and your ZIP code above and the answer changes with them. Do not trust one number for every house.
Is the 500 square feet per ton rule right?
It is a rough guess. It does not know about sun, insulation, air leaks or ducts. A shaded, tight house can need much less. A sunny, leaky one can need more. Use it only to check that an answer is in the right range.
Why does the standard size differ from the exact tons?
Units come in half ton steps, like 2, 2.5 and 3 tons. The exact load rarely lands on a step. Pick the standard size that covers 90 to 115 percent of the load.
What happens if the AC is too big?
It cools the air fast and shuts off. It runs in short bursts and does not pull out much moisture. The house feels cold and damp, and the unit wears faster.
How much airflow does the AC need?
A common rule is about 400 CFM for each ton. This page also shows the supply air from the load itself, using a 20 F drop from the room air to the supply air.
More free tools
An estimate in the style of ACCA Manual J. It does not replace a certified room by room design. Manual J is a trademark of ACCA. HVAC Load is not affiliated with ACCA.