What Size Air Conditioner Do I Need? BTU by Room Size
Bigger is not better with air conditioners. A unit sized to the room cools evenly, removes humidity and costs less to run. Here is how to work out the right number in two minutes.
Quick answer
Allow roughly 20 BTU per square foot as a starting point, then follow the ENERGY STAR chart: about 6,000 BTU for 150 to 250 sq ft, 8,000 for 300 to 350, 10,000 for 400 to 450 and 12,000 for 450 to 550. Add 10 percent for a very sunny room, 600 BTU per extra person beyond two, and 4,000 BTU for a kitchen.
- Difficulty
- Easy DIY
- Rule of thumb
- About 20 BTU per sq ft
- 12,000 BTU covers
- 450 to 550 sq ft
- Sunny room
- Add 10%
- Kitchen
- Add 4,000 BTU
On this page
Air conditioner capacity is measured in BTU per hour, the amount of heat a unit can pull out of a room in an hour. Match that number to the room and the AC runs in long, steady cycles that cool the air and dry it at the same time. Get it wrong in either direction and you pay for it in comfort or energy.
Start with the room’s area
Measure the length and width of the room in feet and multiply them. A 15 by 20 foot living room is 300 square feet. Then find the band on the ENERGY STAR chart:
| Room area | Area in m² | Capacity needed |
|---|---|---|
| 100 to 150 sq ft | 9 to 14 m² | 5,000 BTU |
| 150 to 250 sq ft | 14 to 23 m² | 6,000 BTU |
| 250 to 300 sq ft | 23 to 28 m² | 7,000 BTU |
| 300 to 350 sq ft | 28 to 33 m² | 8,000 BTU |
| 350 to 400 sq ft | 33 to 37 m² | 9,000 BTU |
| 400 to 450 sq ft | 37 to 42 m² | 10,000 BTU |
| 450 to 550 sq ft | 42 to 51 m² | 12,000 BTU |
| 550 to 700 sq ft | 51 to 65 m² | 14,000 BTU |
| 700 to 1,000 sq ft | 65 to 93 m² | 18,000 BTU |
| 1,000 to 1,200 sq ft | 93 to 111 m² | 21,000 BTU |
| 1,200 to 1,400 sq ft | 111 to 130 m² | 23,000 BTU |
| 1,400 to 1,500 sq ft | 130 to 139 m² | 24,000 BTU |
The chart assumes ordinary 8-foot ceilings, average sun, two people and no cooking. If your room is different, adjust.
Adjust for the way the room is used
- Sun.
Subtract 10 percent if the room is heavily shaded. Add 10 percent if it is very sunny, such as a west-facing room with large windows.
- People.
Add 600 BTU for each person beyond two who regularly uses the room. A family room where five people watch TV every evening needs 1,800 BTU more than the chart shows.
- Kitchen.
Add 4,000 BTU if the unit will cool a kitchen. Ovens and cooktops add a lot of heat.
- Round to a real unit.
Air conditioners are sold in steps. Choose the nearest size at or slightly above your number.
Our room AC size calculator does these steps for you and works in square feet or square meters.
Why an oversized AC feels worse
An air conditioner removes moisture by condensing it on the cold indoor coil. That takes time: for the first several minutes of each cycle, the coil is still cooling down and very little water drips off. A unit that is too large drops the temperature to the thermostat setting within a few minutes and shuts off, so it never runs long enough to dry the air. ENERGY STAR describes the result as a room that feels “cold and clammy.”
Short cycles also cost more. Starting a compressor draws a surge of power, and frequent starts wear it out sooner.
Portable air conditioners: read the smaller number
Portable units list two capacities. The larger ASHRAE number is a lab rating that ignores two real-world losses: heat radiating from the exhaust hose back into the room, and hot outdoor air pulled into the house to replace the air the unit blows outside. The Department of Energy’s SACC rating (seasonally adjusted cooling capacity) includes both, and it has been the figure on the yellow EnergyGuide label since 2017.
SACC ratings typically run 25 to 45 percent lower than ASHRAE ratings for the same unit. Size a portable AC by its SACC number, and expect a dual-hose model to cool better than a single-hose model of the same rating.
Check the circuit before you buy a big unit. Most room air conditioners up to about 12,000 BTU run on a standard 115-volt outlet. Larger units may need a dedicated circuit or a 230-volt outlet. The plug type and voltage are listed on the box and in the manual.
Central air and heat pumps need a load calculation
The 20 BTU per square foot rule and the room chart are fine for one room. They are not a reliable way to size a central air conditioner or heat pump for a whole house, because insulation, window area, air leakage, ductwork and climate change the answer dramatically.
Contractors should size whole-house systems with a Manual J load calculation, the industry standard published by the Air Conditioning Contractors of America. Ask to see it. A quote that simply replaces your old system with one the same size, or that is based only on square footage, may repeat an old mistake.
Quick answers for common rooms
| Room | Typical size | Suggested capacity |
|---|---|---|
| Small bedroom or office (10 × 12 ft) | 120 sq ft | 5,000 BTU |
| Primary bedroom (14 × 16 ft) | 224 sq ft | 6,000 BTU |
| Living room (16 × 20 ft) | 320 sq ft | 8,000 BTU |
| Large living room (20 × 22 ft) | 440 sq ft | 10,000 BTU |
| Open kitchen and dining (20 × 25 ft) | 500 sq ft | 12,000 + 4,000 = 16,000 BTU |
| Studio apartment (25 × 25 ft) | 625 sq ft | 14,000 BTU |
Frequently asked questions
How many BTU do I need for a 12x12 room?
A 12 by 12 foot room is 144 square feet, which falls in the 100 to 150 sq ft band of the ENERGY STAR chart: 5,000 BTU. If the room gets strong afternoon sun, step up to 6,000 BTU.
What happens if my air conditioner is too big?
It cools the air quickly and shuts off before it has run long enough to pull moisture out, so the room feels cold and damp. Frequent starts also waste energy and wear the compressor.
What happens if it is too small?
It runs constantly on hot afternoons and never reaches the thermostat setting. In mild weather an undersized unit may be fine, but on the hottest days the room will stay warm.
Why does my portable AC have two BTU ratings?
Portable units list an ASHRAE rating and a lower DOE rating called SACC (seasonally adjusted cooling capacity). SACC accounts for heat from the exhaust hose and the warm air drawn into the room, so it is the better number for sizing.
Sources
- Room Air Conditioners ENERGY STAR (U.S. EPA)
- Room Air Conditioners U.S. Department of Energy, Energy Saver (archived copy, 2025)
- Manual J Residential Load Calculation Air Conditioning Contractors of America
- 16 CFR Part 305: Energy and Water Use Labeling for Consumer Products (Energy Labeling Rule) Electronic Code of Federal Regulations
First published October 6, 2026. Last reviewed October 6, 2026. Spot an error? Tell us and we will check it against the source.