By the contractor rule of thumb, a 1,800 sq ft house in a mixed climate needs about 3.5 tons of air conditioning: 1,800 ÷ 500 sq ft per ton = 3.6 tons, or 43,200 BTU/h. The same rule gives anywhere from 3 to 4.5 tons for that house, and an ACCA Manual J load calculation often lands far lower, which is why this number is a starting point for a quote, not the size to buy.
How the AC size is calculated
The calculator divides the cooled floor area by a square-feet-per-ton figure and rounds to the nearest nominal size. One ton of cooling is 12,000 BTU/h.
Tons = cooled floor area (sq ft) ÷ sq ft per ton; BTU/h = tons × 12,000
The square-feet-per-ton figure comes from two sources:
- Older or average homes: 400 to 600 sq ft per ton. Energy Vanguard reports that contractors "generally use the rule of 1 ton ... for each 400 to 600 square feet." A Florida Solar Energy Center survey found contractors using 350 to 700, with a weighted average of 502.
- Newer, tight homes: 700 to 1,000 sq ft per ton, the modified guideline FSEC cites for better-built houses.
Climate picks the point in the range: hot zones 1 and 2 use the low end (more tons), mixed zones 3 and 4 the middle, cool zones 5 to 7 the high end. That placement is ours; the sources give the range, not a value per zone.
Worked example
| House | Zone | Sq ft per ton | Estimate | Nominal size |
|---|---|---|---|---|
| 1,800 sq ft, older | 3, mixed | 500 | 3.6 tons | 3.5 tons |
| 1,800 sq ft, older | 1, hot | 400 | 4.5 tons | 4 tons |
| 1,800 sq ft, older | 5, cool | 600 | 3 tons | 3 tons |
| 1,800 sq ft, newer | 3, mixed | 850 | 2.1 tons | 2 tons |
When an estimate falls exactly between two sizes, the calculator picks the smaller one, because oversizing is the more common and more costly mistake.
AC BTU calculator for the whole house
To read the answer in BTU, multiply tons by 12,000: 3.5 tons is 42,000 BTU/h. Nominal central units come in 18,000 to 60,000 BTU/h steps (1.5, 2, 2.5, 3, 3.5, 4 and 5 tons in the ENERGY STAR list of certified heat pumps). A single room needs the BTU calculator instead, which uses the ENERGY STAR room chart.
Room by room
The second tab adds the rooms up with the ENERGY STAR room chart, including sun, people and kitchens. With the defaults (a sunny 320 sq ft living room with four people, a 180 sq ft kitchen and bedrooms of 200 and 150 sq ft) the rooms add to 32,000 BTU/h, 2.67 tons for 850 sq ft. The square footage rule would call that floor area 1.7 tons.
The gap is expected. The room chart sizes each room for its own worst hour and puts a 5,000 BTU/h floor under small rooms, so the sum is an upper bound. It is useful for checking that no room is starved, and for splitting airflow later in the ductwork calculator.
Why oversizing is the problem to avoid
FSEC found that over a third of contractors oversize on purpose and that in one Florida study more than half the systems exceeded 120% of the Manual J load. Oversized systems run short cycles, so they remove less moisture and spend more time in their inefficient start-up phase; FSEC puts the energy penalty at about 10% for a system 50% too big. The oversizing check under the result compares the nominal size with the estimate and with Energy Vanguard's Manual J data, where 75 homes averaged about 1,200 sq ft per ton. If your house is newer or well sealed, that line tells you a load calculation will probably come in smaller.
What a Manual J adds
Manual J (ACCA) works out the heat gain through each wall, window, ceiling and duct, plus air leakage and people, at your county's design temperature. Most building departments expect it for new equipment. Ask the contractor for the report and compare its total with this estimate: a quote well above the rule of thumb deserves a question.
Next steps
- Airflow: at 400 CFM per ton (Trane), 3.5 tons moves 1,400 CFM. The CFM calculator converts other sizes.
- Ducts: size the trunk and branches for that airflow with the duct size calculator.
- Heat: the furnace size calculator sizes the furnace for the same floor area and zone.
- No ducts: a mini split size calculator layout sizes one head per room.
If you searched for an air conditioner BTU calculator, this is it: enter the room and get the BTU rating to shop for, then compare with the BTU calculator for heating.