A common starting estimate is about 20 BTU of cooling per square foot, which means roughly 1 ton (12,000 BTU) for every 500 to 600 square feet. That’s only a starting point. The correct size depends on your climate, insulation, windows, ceiling height and how many people live in the home. Contractors confirm it with a load calculation called Manual J.
Quick answers
- 1 ton of cooling = 12,000 BTU per hour.
- A 1,500 sq ft home often lands between 2.5 and 3 tons.
- Hot, sunny climates and poor insulation push the size up; tight, shaded homes push it down.
- Bigger is not better: an oversized AC short-cycles and leaves the house humid.
- Always ask your contractor for a Manual J load calculation before replacing a system.
Quick sizing chart
| Home size (sq ft) | Approximate BTU | Approximate tons |
|---|---|---|
| 600–1,000 | 18,000–21,000 | 1.5–2 |
| 1,000–1,300 | 21,000–24,000 | 2 |
| 1,300–1,600 | 24,000–30,000 | 2–2.5 |
| 1,600–1,900 | 30,000–36,000 | 2.5–3 |
| 1,900–2,200 | 36,000–42,000 | 3–3.5 |
| 2,200–2,600 | 42,000–48,000 | 3.5–4 |
| 2,600–3,200 | 48,000–60,000 | 4–5 |
Residential central air conditioners commonly come in half-ton steps from 1.5 to 5 tons. Homes larger than about 3,000 square feet often use two systems or a zoned system rather than one very large unit.
How to adjust the estimate
Start with the chart, then adjust:
- Hot climate (Southwest, Gulf Coast, Florida): add about 10% to 15%.
- Mild or cool climate (Pacific Northwest, northern states): subtract about 10%.
- Lots of west- or south-facing windows: add 10%.
- Ceilings over 8 feet: add about 10% for every extra 2 feet of height.
- Recent insulation and air sealing upgrades: subtract 10% to 20%.
- More than 2 regular occupants: add about 600 BTU per extra person.
- Kitchen in the cooled area: add about 4,000 BTU.
Why oversizing is a problem
It’s tempting to buy a larger unit “just in case.” But an oversized air conditioner:
- Short-cycles. It cools the air quickly and shuts off before running long enough to remove humidity. The house feels cool but clammy.
- Wears out sooner. Starting is the hardest moment for a compressor. More starts mean more wear.
- Costs more to buy and can need larger ductwork to perform properly.
- Creates uneven temperatures, because rooms far from the air handler don’t get enough run time.
Why undersizing is a problem
An undersized system runs nonstop on hot days and still can’t hold the set temperature. It keeps humidity low because it runs long, but your bills rise and comfort suffers during heat waves.
What a Manual J load calculation includes
Manual J is the industry standard method published by the Air Conditioning Contractors of America (ACCA). It accounts for:
- Square footage and ceiling height of each room
- Window size, type and direction
- Insulation levels in walls, attic and floors
- Air leakage
- Local design temperatures
- Occupants and appliances
A careful contractor measures the home or uses accurate plans. If a quote simply matches the size of your old unit, ask how they confirmed it. The old system may have been the wrong size from the start. See How to Compare HVAC Quotes.
Sizing for mini-splits and room units
Ductless mini-splits are sized per room or zone. A typical bedroom of 150 to 250 square feet often needs a 6,000 to 9,000 BTU head, and an open living area of 400 to 600 square feet may need 12,000 to 18,000 BTU. Window units follow similar per-room math.
Size and efficiency are different things
Size (tons) is how much heat the system removes per hour. Efficiency (SEER2) is how much electricity it uses to do it. A correctly sized, mid-efficiency system usually performs better than an oversized high-efficiency one. Learn more in SEER2 Ratings Explained.
Frequently asked questions
Is 3 tons enough for 2,000 square feet?
It can be in a well-insulated home in a mild or moderate climate. In a hot climate with lots of sun exposure, 3.5 tons may be needed. A load calculation settles it.
Should my new AC be the same size as the old one?
Not necessarily. If you’ve added insulation, replaced windows or the original was oversized, a smaller system may be better.
Does the furnace need to match the AC size?
The air handler or furnace blower must move enough air for the AC size, usually around 400 cubic feet per minute per ton. Your installer should confirm the indoor and outdoor equipment are matched.