What Size Air Conditioner Do I Need? (Tons by Square Foot)
Updated February 25, 2026
Just want the number? Use the AC and mini-split sizing calculator.
A good starting estimate is about 20 BTU per hour of cooling per square foot of living space, which works out to roughly 2.5 tons for a 1,500 square foot home. Air conditioners are sold in tons, and one ton equals 12,000 BTU per hour, so you divide your total BTU by 12,000 to get the size. The trick is that square footage is only the first input. Sun, occupancy, and how leaky your house is all move the number.
Here is how to go from a rough rule of thumb to the right size for your house.
The quick by-size table
Using the 20 BTU per square foot rule, the math lands roughly here:
- 1,000 sq ft: about 20,000 BTU, or 1.5 tons
- 1,500 sq ft: about 30,000 BTU, or 2.5 tons
- 2,000 sq ft: about 40,000 BTU, or 3 to 3.5 tons
These are starting points, not final answers. The 20 BTU figure assumes a reasonably insulated home in a moderate climate. A hot, sunny region pushes the number up. A shaded, tightly sealed home pulls it down.
What moves the number
Two homes of the same size can need different units. The factors that matter most:
- Sun exposure. A room with large west or south facing windows takes on a lot of afternoon heat. Heavy glass and direct sun can add 10 percent or more to the load. A shaded north side needs less.
- Occupancy. People give off heat, roughly 230 BTU per hour each at rest, so a busy household adds load. So do kitchens, since the oven and stove dump heat into the space.
- Ceiling height and insulation. You cool volume, not floor area, so vaulted ceilings raise the load. A drafty, poorly insulated house pushes toward the high end of the range.
The U.S. Department of Energy publishes guidance on sizing room air conditioners that walks through these same adjustments.
Why bigger is not better
It is tempting to round up to feel safe on the hottest day. Resist it. An oversized air conditioner cools the air fast, hits the thermostat target, and shuts off, a pattern called short cycling. That sounds fine, but it creates two real problems.
First, it never runs long enough to pull moisture out of the air. Dehumidifying happens while the unit runs, so a short cycle leaves the house cool but clammy. The Environmental Protection Agency notes that indoor humidity above about 60 percent encourages mold and dust mites, which is exactly what you are trying to avoid.
Second, all that stopping and starting wears out the compressor faster and uses more energy than a right sized unit that runs in longer, steady cycles. Right sizing, not maximum size, is the goal.
Central AC versus a mini-split
If you are cooling a single room, an addition, or a house without ductwork, a ductless mini-split is often the better fit. You size it the same way, by BTU per square foot, but each indoor head is matched to one zone rather than the whole house. That avoids the oversizing trap, since you are not forcing one large unit to serve rooms with very different loads.
For a whole house with existing ducts, central AC is usually the simpler swap. Either way, the sizing math starts from the same BTU estimate. What size mini-split do I need for one room covers the head sizes and the adjustments that move you between them.
Get your number
Plug your square footage, climate, sun exposure, and insulation into the AC and mini-split sizing calculator and it returns a BTU and tonnage estimate with the math shown. If you want to see how the cooling load fits into your whole house, the whole-home heat load calculator covers both heating and cooling in one pass.
For a critical install, ask your contractor for a room-by-room Manual-J load calculation before you buy, and use the estimate here to check that a quote is not oversized. Run your own numbers in the calculator below.