Quick answer
For one room, use the ENERGY STAR chart: a 150 to 250 sq ft room needs a 6,000 BTU per hour unit, and a 450 to 550 sq ft room needs 12,000 (one ton). For a whole house, ENERGY STAR says your contractor should size the system with ACCA Manual J, a calculation of the heating and cooling your home needs, not a square-foot rule.
Sizing a window or portable AC
ENERGY STAR sizes room air conditioners by the floor area they cool, assuming an 8 ft ceiling:
| Room area | Capacity needed |
|---|---|
| 100 up to 150 sq ft | 5,000 BTU per hour |
| 150 up to 250 sq ft | 6,000 BTU per hour |
| 250 up to 300 sq ft | 7,000 BTU per hour |
| 300 up to 350 sq ft | 8,000 BTU per hour |
| 350 up to 400 sq ft | 9,000 BTU per hour |
| 400 up to 450 sq ft | 10,000 BTU per hour |
| 450 up to 550 sq ft | 12,000 BTU per hour |
| 550 up to 700 sq ft | 14,000 BTU per hour |
| 700 up to 1,000 sq ft | 18,000 BTU per hour |
| 1,000 up to 1,200 sq ft | 21,000 BTU per hour |
| 1,200 up to 1,400 sq ft | 23,000 BTU per hour |
| 1,400 up to 1,500 sq ft | 24,000 BTU per hour |
| 1,500 up to 2,000 sq ft | 30,000 BTU per hour |
| 2,000 up to 2,500 sq ft | 34,000 BTU per hour |
Then adjust: reduce capacity by 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU for each person beyond two who regularly uses the room, and add 4,000 BTU for a kitchen. The AC BTU calculator applies the chart and these adjustments for you.
Sizing central air or a heat pump
ENERGY STAR says your contractor should verify the right size "using a tool called 'Manual J'", a calculation of how much heating and cooling your home needs. ACCA, which publishes it, calls Manual J the national ANSI-recognized standard for sizing loads in homes, and says a load calculation done this way is required by national building codes and most state and local jurisdictions. The equipment is then chosen to match that load with ACCA Manual S.
The square-foot rule you may hear is one ton (12,000 BTU per hour) for every 400 to 500 sq ft. The Department of Energy calls that "a commonly accepted old estimate" that "does not take into account local climate and how well a house is sealed and insulated", notes that in a very efficient house one ton can condition 800 to 1,000 sq ft, and tells homeowners to insist on Manual J. Mississippi State University Extension puts it more bluntly: "Don't use a contractor who wants to size your system solely on the square footage of your house."
Why bigger is not better
- Comfort: ENERGY STAR (for room units) and Mississippi State University Extension (for central systems) both say an oversized unit cools before it can remove enough humidity, leaving the house "cool but clammy", in MSU's words.
- Wear: ENERGY STAR says oversized equipment may cycle too frequently, which means less comfort and a shorter life.
- Cost and air quality: DOE says oversized cooling systems can be noisy and lead to mold and poor indoor air quality, while costing too much.
Heat pumps in cold climates
The Northeast Energy Efficiency Partnerships (NEEP) cold-climate guide, hosted by DOE, says to size a heat pump to meet both the heating and the cooling load, with capacity checked at your area's design temperature. ENERGY STAR's cold-climate designation requires testing down to 5°F, with a coefficient of performance of at least 1.75 at 5°F and at least 70% of its 47°F heating capacity. ENERGY STAR notes that a cold-climate heat pump keeps working below 5°F, but pairing it with backup heat is the most efficient approach when it gets colder.
The NEEP guide gives two approaches for replacing a whole system: match 100% to 115% of the estimated heating load at the design temperature without auxiliary heat, or cover 75% to 85% of it and add just enough supplemental heat to make up the difference. It also warns against padding the load estimate with extra safety factors.
Tons and BTUs
One ton of cooling is 12,000 BTU per hour, according to Mississippi State University Extension, so a 3-ton system is rated at 36,000 BTU per hour.
Frequently asked questions
What size AC do I need for 1,000 square feet?
For a room air conditioner, ENERGY STAR's chart gives 18,000 BTU per hour for rooms just under 1,000 sq ft and 21,000 for 1,000 up to 1,200 sq ft, before adjusting for sun, shade, people and a kitchen. For central air in a 1,000 sq ft home, the size should come from a Manual J load calculation; DOE says the old rule of one ton per 400 to 500 sq ft ignores climate and insulation.
How many square feet does a ton of AC cool?
There is no fixed figure. DOE calls one ton per 400 to 500 sq ft an old estimate, and says a very efficient house can need only one ton per 800 to 1,000 sq ft. A Manual J calculation gives the answer for your house.
Is it bad to oversize an air conditioner?
Yes. ENERGY STAR says an oversized unit cools the air before removing enough humidity, leaving the room damp and clammy, and may cycle too frequently, shortening its life.
How many BTU is a ton?
One ton of cooling is 12,000 BTU per hour.
Calculators for this
AC BTU
What size air conditioner do you need?
Heating Cost Comparison
Compare the cost of heat from natural gas, propane, heating oil, electric heat and a heat pump, per million Btu and for a whole winter, with your own prices.
Insulation
Get batt bags for walls, floors and attics, or bags of blown-in fiberglass or cellulose to reach your target R-value, by attic size.
Published 2026-11-11. Sources: ENERGY STAR, Room Air Conditioners (sizing chart); ENERGY STAR, Air-Source Heat Pumps; ENERGY STAR, Air-Source Heat Pumps key product criteria; ENERGY STAR, HVAC Quality Installation; U.S. Department of Energy, Guide to Home Heating and Cooling (PDF); Mississippi State University Extension, P2785 (PDF); ACCA, Manual J; ACCA, Manual S; NEEP, Guide to Sizing and Selecting Air-Source Heat Pumps in Cold Climates (hosted by DOE, PDF). See how we calculate.