How to Choose the Right Air Conditioner Size for a Room
Why bigger isn't automatically better when sizing an AC unit
What a BTU rating means
Air conditioners are rated by how much heat they can remove per hour, in British Thermal Units per hour (BTU/h, usually shortened to BTU). One BTU is the heat needed to raise one pound of water by one degree Fahrenheit. Central systems are often described in tons instead: one ton of cooling is 12,000 BTU/h, so a '2-ton' system is 24,000 BTU/h.
Room units — window, through-the-wall and portable air conditioners — typically range from about 5,000 to 24,000 BTU. The goal is to match that capacity to the room's heat gain as closely as practical.
Start with ENERGY STAR's room-size chart
Room air conditioners are sized by floor area first, then adjusted for the room's conditions. ENERGY STAR publishes a chart of the cooling capacity needed for each band of room area. Its capacities assume an 8 ft ceiling, and they are a starting estimate, not a substitute for a proper load calculation on larger or more complex jobs.
To use the chart, measure the room's floor area (length × width), find the band it falls in, and read the capacity. A 15 × 25 ft room is 375 ft², in the 350–400 ft² band, so the starting capacity is 9,000 BTU per hour.
| Area to be cooled | Capacity needed |
|---|---|
| 100–150 ft² | 5,000 BTU/h |
| 150–250 ft² | 6,000 BTU/h |
| 250–300 ft² | 7,000 BTU/h |
| 300–350 ft² | 8,000 BTU/h |
| 350–400 ft² | 9,000 BTU/h |
| 400–450 ft² | 10,000 BTU/h |
| 450–550 ft² | 12,000 BTU/h |
| 550–700 ft² | 14,000 BTU/h |
| 700–1,000 ft² | 18,000 BTU/h |
| 1,000–1,200 ft² | 21,000 BTU/h |
| 1,200–1,400 ft² | 23,000 BTU/h |
| 1,400–1,500 ft² | 24,000 BTU/h |
| 1,500–2,000 ft² | 30,000 BTU/h |
| 2,000–2,500 ft² | 34,000 BTU/h |
How the calculator's 20 BTU/ft² rule compares
The Air Conditioner BTU Calculator uses a simpler rule of thumb of its own: about 20 BTU per square foot of floor area, then the same adjustments, rounded up to the next size in its list of common units (5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 21,000, 23,000 and 24,000 BTU). That 20 BTU/ft² figure is not ENERGY STAR's method, and the two don't always agree.
For many rooms the result is the same, but in several bands between 150 and 550 ft² the chart calls for one step more than the calculator, and the calculator's size list has no 7,000 or 9,000 BTU step. If your room is in one of those bands, check it against the chart, or use the calculator's optional safety margin.
| Room area | ENERGY STAR chart | Calculator: 20 BTU/ft² load | Calculator's recommended size |
|---|---|---|---|
| 120 ft² | 5,000 BTU/h | 2,400 BTU | 5,000 BTU |
| 200 ft² | 6,000 BTU/h | 4,000 BTU | 5,000 BTU |
| 275 ft² | 7,000 BTU/h | 5,500 BTU | 6,000 BTU |
| 325 ft² | 8,000 BTU/h | 6,500 BTU | 8,000 BTU |
| 375 ft² | 9,000 BTU/h | 7,500 BTU | 8,000 BTU |
| 425 ft² | 10,000 BTU/h | 8,500 BTU | 10,000 BTU |
| 500 ft² | 12,000 BTU/h | 10,000 BTU | 10,000 BTU |
| 650 ft² | 14,000 BTU/h | 13,000 BTU | 14,000 BTU |
| 850 ft² | 18,000 BTU/h | 17,000 BTU | 18,000 BTU |
What pushes the estimate up or down
- Ceiling height — taller ceilings mean more air volume to cool for the same floor area
- Sun exposure — a room with large west-facing windows generally needs more cooling capacity than a similarly-sized shaded room
- Occupancy — more people in a room adds body heat load, relevant for rooms that are regularly crowded
- Heat-generating equipment — a kitchen or a room with significant electronics adds load beyond the base estimate
The standard adjustments
ENERGY STAR turns the most common of these into simple numbers, and the calculator applies the same ones: reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny one, add 600 BTU for each regular occupant beyond two, and add 4,000 BTU if the unit will cool a kitchen. Ceiling height, poor insulation and large windows aren't in the chart — it assumes an 8 ft ceiling — so lean toward the next size up if they apply.
Worked example
A 14 × 20 ft family room (280 ft²) gets strong afternoon sun and is used by three people most evenings. On the ENERGY STAR chart, 280 ft² is in the 250–300 ft² band: 7,000 BTU/h. Very sunny: 7,000 × 1.1 = 7,700 BTU/h. One occupant beyond two: + 600, for 8,300 BTU/h. Choose a unit rated at or above that; the chart's next step is 9,000 BTU/h.
The calculator gets a smaller figure for the same room: 280 × 20 × 1.1 = 6,160 BTU, plus 600 for the extra person = 6,760 BTU, which rounds up to an 8,000 BTU unit. That gap is the one described above, so in this case the chart's figure is the safer one to buy to.
The calculator's own example is a 300 ft² living room with average sun and four regular occupants: 6,000 BTU base plus 1,200 BTU for the two extra people gives 7,200 BTU, which points to an 8,000 BTU unit.
Why oversizing causes its own problems
It's tempting to assume a bigger unit is simply 'safer,' but an oversized air conditioner cools the room to the thermostat setpoint quickly and then shuts off, without running long enough to properly dehumidify the air. This can leave a room feeling cold but clammy, and the frequent on/off cycling is also harder on the equipment over time than a properly-sized unit running longer, steadier cycles.
ENERGY STAR's own warning is that an oversized air conditioner costs more to buy, wastes energy and does not cool any better. That's why the calculator picks the smallest standard size that meets the load rather than adding a percentage on top.
And why undersizing is the more common mistake
An undersized unit runs constantly on hot afternoons and never reaches the setpoint. It happens most often when the room is sunnier, busier or less insulated than assumed — or when the unit is expected to cool adjoining spaces through an open doorway. If a room is borderline between two sizes and has any of those factors, the larger of the two is usually the better choice.
Room units vs. whole-house systems
The square-foot method is meant for single rooms. Central air conditioners and heat pumps should be sized with a full heat-gain calculation — ACCA Manual J in the US — which accounts for insulation levels, window area and orientation, air leakage, duct location and local climate. 'Square feet per ton' rules of thumb for whole houses are unreliable and a common cause of oversized systems.
Improving the building often shrinks the equipment you need. Better insulation and air sealing reduce heat gain; the guide to calculating insulation requirements and the Insulation Calculator help estimate that work.
Other things to check before buying
- Efficiency — room units are rated by CEER; central systems by SEER2 in the US since 2023. Higher numbers use less electricity for the same cooling.
- Electrical supply — larger room units may need a dedicated circuit or a 230 V outlet; check the unit's plug type and the circuit it will use.
- Window or wall fit — measure the opening, and check sash clearance for window units.
- Portable units — their rated capacity can be optimistic in practice because of heat and air leakage through the exhaust hose, so compare the newer seasonally adjusted ratings where available.
Common mistakes
- Buying the biggest unit that fits the window.
- Ignoring a kitchen's extra heat load.
- Using a room-sizing rule for a whole house.
- Expecting one room unit to cool several rooms through doorways.
FAQ
- What size air conditioner do I need for a 12x12 room?
- A 12 × 12 ft room is 144 ft², in ENERGY STAR's 100–150 ft² band, which calls for 5,000 BTU/h with an 8 ft ceiling. Add 4,000 BTU if it's a kitchen, and adjust by 10% for heavy shade or strong sun. The calculator's 20 BTU/ft² rule gives 2,880 BTU, which also rounds up to a 5,000 BTU unit.
- What size room will a 6,000 BTU air conditioner cool?
- On ENERGY STAR's chart, 6,000 BTU/h is the capacity for a room of 150–250 ft² with an 8 ft ceiling, before adjustments. Strong sun, more than two regular occupants or a kitchen push a room of that size above 6,000 BTU.
- How many BTU do I need per square foot?
- ENERGY STAR doesn't use a single per-square-foot figure; it gives a capacity for each room-size band, such as 10,000 BTU/h for 400–450 ft². The Air Conditioner BTU Calculator uses about 20 BTU per square foot as its own rule of thumb, which matches the chart in some bands and comes out one size smaller in others.
- Is it bad to oversize an air conditioner?
- Yes. An oversized unit short-cycles, removing less humidity, which leaves the room cold and clammy, and the frequent starts add wear. Choose the smallest standard size that meets the calculated load.
- How many BTU is a ton of air conditioning?
- One ton of cooling equals 12,000 BTU per hour. A 24,000 BTU unit is a 2-ton unit.
Related Calculators
Related Guides
More From the Blog
Sources
ENERGY STAR, Room Air Conditioners
Capacity needed by room area (based on an 8 ft ceiling), the shade, sun, occupancy and kitchen adjustments, and the oversizing warning.
ENERGY STAR, Room Air Conditioners — referenceACCA Manual J
Residential load calculation for whole-house heating and cooling equipment sizing.