How to Size an Air Conditioner for Your Home

BTU per hour, tons and the quick estimate

Air conditioners are rated in BTU per hour, the heat they can pull out of the house in an hour, and in tons. One ton of cooling is 12,000 BTU/h; the number comes from the heat it takes to melt a ton of ice in a day. Residential split systems run from 1.5 to 5 tons, mostly in half-ton steps, so 18,000 to 60,000 BTU/h.

The quick estimate most people start from is floor area times a BTU per square foot figure: roughly 20 BTU/h per square foot in a mild climate, 25 in a moderate one like the Midwest or mid-Atlantic, 30 in the hot Southeast and Texas, and up to 35 in the desert Southwest. Adjust up for ceilings over 8 ft, poor insulation and heavy sun, add about 600 BTU/h for each person beyond two, and 4,000 BTU/h for a kitchen. The AC sizing calculator runs those factors and rounds to the nearest half ton.

Why the square-foot rule is only a start, and why bigger is worse

Two houses with the same floor area can have cooling loads a ton apart. The rule of thumb does not know which way the big windows face, whether the attic has R-19 or R-49, whether the ducts run through a 130°F attic, how leaky the envelope is, or what the design temperature is on the hottest afternoons where you live. It treats a west-facing room with a glass wall the same as a shaded bedroom.

Use the estimate to sanity-check a proposal. If a contractor quotes a 5-ton unit on a 1,600 sq ft house in Indiana, ask to see the load calculation.

Oversizing is the more common mistake. An oversized air conditioner satisfies the thermostat fast and shuts off. That short cycling costs you three ways:

• Humidity. A coil needs long run times to pull moisture out of the air. Short cycles leave the house cool but clammy, which is the complaint behind many "the AC is too big" service calls.
• Wear. Starts are the hardest part of a compressor's life. More starts per hour means more wear on the compressor, contactor and capacitor.
• Comfort and cost. Short cycles never move air long enough to even out the far rooms, and a bigger unit costs more and often needs bigger ducts and a larger circuit.

Undersizing has its own problem, a house that never quite gets there on the hottest days. But a unit that runs long and steady on a design day is the goal, not a defect.

What Manual J, S and D actually do

ACCA Manual J (8th edition) is the residential load calculation. The 2021 IRC (M1401.3) requires equipment to be sized from it or an approved equivalent. It walks the house room by room and accounts for the envelope (wall, roof, floor and window areas and their U-values), window orientation, shading and solar gain, infiltration from a blower-door test or an estimate, duct location and leakage, internal gains from people and appliances, and the local design temperatures for summer and winter. The result is a sensible load and a latent load in BTU/h, not just a total.

Manual S then picks the equipment: a unit whose rated capacity at your design conditions covers both the sensible and latent loads without going far over. Manual S caps the oversizing, roughly 115% of the total load for a single-stage unit, with more room for two-stage and variable-speed equipment that can run at part load. Manual D sizes the ducts to deliver the airflow each room needs, about 400 CFM per ton overall, at a static pressure the blower can handle. The CFM to BTU calculator and the duct size calculator cover the arithmetic behind both.

Window units, SEER2 and refrigerants

Window and portable units are sized the same way at the room level: about 20 BTU/h per square foot, 10% more for a sunny room, 10% less for a shaded one, 600 BTU/h per extra person and 4,000 for a kitchen. A 12 × 15 ft bedroom is 180 sq ft, about 3,600 BTU/h, so the smallest common unit at 5,000 BTU/h is already more than enough. Oversizing hurts here too.

Efficiency has been rated in SEER2 since 2023, a tougher test run at higher external static pressure than the old SEER. SEER2 numbers come out a few percent lower than SEER for the same unit, so do not compare one to the other directly. The federal minimums for most new residential split systems under 45,000 BTU/h are 13.4 SEER2 in the North and 14.3 SEER2 in the South and Southwest. New equipment built from 2025 on uses lower-GWP A2L refrigerants such as R-454B and R-32 in place of R-410A, which changes the service tools and line-set rules but not the sizing.

Worked example

A 1,500 sq ft ranch in northern Indiana: moderate climate, 8 ft ceilings, three people, average insulation and sun. Rule of thumb: 1,500 × 25 = 37,500 BTU/h, plus 600 for the third person, about 38,000 BTU/h. That rounds to 3 tons (36,000 BTU/h), a reasonable first guess.

Now the Manual J. The house has new low solar gain windows, R-49 in the attic, ducts in the basement instead of the attic, and a blower-door number that says it is reasonably tight. The calculation comes back at 28,000 BTU/h total, about 24,000 of it sensible. Manual S matches that to a 2.5-ton unit (30,000 BTU/h), which covers the load with a 7% margin. The 3-ton rule-of-thumb unit would have been 29% oversized, cooling the house fast and leaving it humid.

Checking the numbers in the field

Once the unit is in, the sizing shows up in the running numbers: a coil temperature drop typically in the 15 to 20°F range, airflow near 400 CFM per ton, and a run time on a hot afternoon that is long and steady. AirForge carries the CFM, BTU and refrigerant tools for those checks on the rooftop with no signal, and the HVAC practice exam covers load calculation questions if you are studying for a certification.