For one room, use ENERGY STAR's sizing chart: about 6,000 BTU for 150–250 sq ft, 10,000 BTU for 400–450 sq ft and 14,000 BTU for 550–700 sq ft, then adjust for sun, shade, extra people and kitchens. Central air and mini-splits should be sized with a Manual J load calculation, not square footage alone.
Key facts
- ENERGY STAR's room AC chart runs from 5,000 BTU (100–150 sq ft) to 34,000 BTU (2,000–2,500 sq ft) and assumes an 8-foot ceiling.
- Adjust the chart: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU per regular occupant beyond two, +4,000 BTU in a kitchen.
- One ton of cooling is 12,000 BTU per hour.
- ENERGY STAR says an oversized unit costs more, wastes energy and leaves the room damp and clammy.
- Contractors' rule of thumb of 400–600 sq ft per ton tends to oversize; ENERGY STAR and DOE point to Manual J.
What size air conditioner do I need for my room?
Room air conditioners (window, through-the-wall and similar units) are rated in BTU per hour of cooling. ENERGY STAR publishes a chart that matches room area to capacity. Measure the room's length and width, multiply to get square feet, and find the row:
| Room area (sq ft) | Cooling capacity (BTU/hr) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
The chart assumes an 8-foot ceiling, so a room with a higher ceiling holds more air than the chart allows for. Our BTU calculator applies the same chart and adjustments for you.
How many BTU per square foot?
The chart is not a single fixed ratio. Divide each capacity by the top of its range and you get roughly 20–24 BTU per sq ft for typical bedrooms and living rooms, falling to about 14–18 BTU per sq ft for very large open spaces, where one unit covers more floor. That is why "20 BTU per square foot" is a reasonable mental check for a single room but not a sizing method.
How do I adjust the BTU for sun, shade, people and kitchens?
ENERGY STAR lists four adjustments to the chart value:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people: add 600 BTU for each person beyond two who regularly uses the room.
- Kitchen: add 4,000 BTU, because cooking appliances add heat.
ENERGY STAR also suggests that if the unit will sit near a corner of the room, you choose one that can aim its airflow where you need it.
Worked examples
How many BTU for a 400 sq ft bedroom?
A 400 sq ft room sits on the line between two rows. ENERGY STAR's ranges read "350 up to 400" and "400 up to 450," so 400 sq ft falls in the 10,000 BTU row. If the bedroom is heavily shaded, take off 10%: 10,000 × 0.9 = 9,000 BTU, which matches the row just below. See the 400 sq ft AC size page for more detail.
How many BTU for a sunny 600 sq ft living room?
600 sq ft is in the 550–700 row: 14,000 BTU. A very sunny room adds 10%: 14,000 × 1.1 = 15,400 BTU. If four people regularly use the room, add 600 BTU for each of the two people beyond the first two: 15,400 + 1,200 = 16,600 BTU. Look for a unit close to that figure rather than jumping far above it. The 600 sq ft page walks through other combinations.
How many BTU for a kitchen?
Take a 300 sq ft kitchen. The chart gives 8,000 BTU for 300 up to 350 sq ft. Add 4,000 BTU for cooking heat: 8,000 + 4,000 = 12,000 BTU. A 12,000 BTU unit would normally cover a 450–550 sq ft room, which shows how much heat a kitchen adds.
How many BTU for 1,000 sq ft?
1,000 sq ft is another boundary: the 700 up to 1,000 row gives 18,000 BTU and the 1,000 up to 1,200 row gives 21,000 BTU. A space that size is usually several rooms, though, and the chart is meant for a single room. For a whole apartment or small house, read the central AC and mini-split sections below and the 1,000 sq ft page.
Why is an oversized air conditioner bad?
A bigger unit than you need is not "extra insurance." ENERGY STAR says an oversized air conditioner costs more to buy, wastes energy and does not cool any better. It also cools the room down before it has run long enough to pull moisture out of the air, so the room ends up feeling damp and clammy.
The DOE's Building Science Education program adds that oversized systems switch on and off too often, known as short cycling, which can hurt both comfort and efficiency. An undersized unit has the opposite problem: it runs constantly and still may not reach your setpoint on the hottest days. The goal is a unit that matches the room.
What size central air conditioner do I need?
Central air conditioners and heat pumps are sold in tons. One ton equals 12,000 BTU per hour, so:
| System size | Cooling capacity (BTU/hr) |
|---|---|
| 1.5 ton | 18,000 |
| 2 ton | 24,000 |
| 2.5 ton | 30,000 |
| 3 ton | 36,000 |
| 4 ton | 48,000 |
| 5 ton | 60,000 |
How many square feet per ton?
Many contractors size by a rule of thumb of 400–600 sq ft per ton. By that rule, a 2,000 sq ft house would get a system of about 3.3 to 5 tons. Building scientist Allison Bailes, writing on the ACCA's HVAC blog, says this tends to oversize. In 40 Manual J calculations for homes in hot and mixed climates, he found an average of 1,431 sq ft per ton, and he says a result under 1,000 sq ft per ton is likely to be wrong. The DOE's Building Science Education program likewise says rules of thumb are too often used and produce excessively oversized systems.
The ton count on a home depends on insulation, windows, air leakage, duct location, orientation and local climate, not just floor area. Two houses of the same size can need very different systems.
What is a Manual J load calculation?
Manual J is the Air Conditioning Contractors of America (ACCA) method for calculating how much heating and cooling a specific house needs. A contractor enters the home's dimensions, insulation levels, window sizes and directions, air tightness and local design temperatures. Ask any contractor quoting a new system whether they did a Manual J for your house. If the answer is a square-foot rule, get another quote.
What size mini split do I need?
Ductless mini-splits are rated in BTU per hour too, and the same 12,000 BTU = 1 ton conversion applies: a 9,000 BTU head is 0.75 ton, 12,000 BTU is 1 ton, 18,000 BTU is 1.5 tons and 24,000 BTU is 2 tons.
There is no official square-foot chart for mini-splits. ENERGY STAR says your contractor should verify the system size with Manual J. ENERGY STAR describes Manual J as a calculation of both the heating and the cooling a home needs, which matters because a mini-split heat pump does both jobs. For a single room, the room AC chart above gives a rough sense of the summer cooling load only.
Does the right size affect what it costs to run?
Yes. Larger units draw more watts, and an oversized one cycles inefficiently. A typical 8,000 BTU window unit uses about 660 W while running; see how many watts a window air conditioner uses and how much it costs to run an air conditioner. If you're not sure what the numbers on the box mean, start with what a BTU is.
Questions people ask
How many BTU do I need per square foot?
ENERGY STAR's room chart works out to roughly 20–24 BTU per square foot for typical rooms and less for very large spaces. Use the chart row for your room's area, then adjust for sun, shade, people and kitchens.
How many BTU are in a ton?
One ton of cooling is 12,000 BTU per hour, so a 3-ton central air conditioner delivers 36,000 BTU per hour.
What size air conditioner do I need for a 2,000 sq ft house?
It depends on the house, which is why a Manual J load calculation is the right answer. The common 400–600 sq ft per ton rule would suggest about 3.3–5 tons, but Manual J results often come in well below that.
What size mini split do I need for one room?
Have a contractor verify it with Manual J, as ENERGY STAR advises. The room AC chart gives a rough cooling figure, but a mini-split heat pump also heats, and Manual J accounts for both.
Is it better to get a bigger air conditioner?
No. ENERGY STAR says an oversized unit costs more, wastes energy and leaves the room damp and clammy because it cools before it removes humidity.
