What size air conditioner do I need?
Room air conditioners are sold by BTUs, and the right number depends mostly on the room's size — with a few adjustments that make a real difference.
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The quick answer
For a typical room with 8 ft ceilings, ENERGY STAR's sizing chart runs from 5,000 BTU per hour for a small bedroom to 12,000 BTU/h for a room of about 500 square feet. Small rooms need more BTU per square foot than large ones, so a single "BTU per square foot" rule doesn't work well. Measure the room, find its band in the chart, then adjust for sun, people and cooking. The BTU calculator does all of it for you.
| Room | Area | Capacity (ENERGY STAR chart) |
|---|---|---|
| Small bedroom, 10 × 12 ft | 120 sq ft | 5,000 BTU/h |
| Bedroom, 12 × 14 ft | 168 sq ft | 6,000 BTU/h |
| Large bedroom, 16 × 18 ft | 288 sq ft | 7,000 BTU/h |
| Living room, 18 × 20 ft | 360 sq ft | 9,000 BTU/h |
| Open living area, 20 × 25 ft | 500 sq ft | 12,000 BTU/h |
The four adjustments
- Sun: a heavily shaded room needs about 10% less; a very sunny one about 10% more.
- People: each person gives off heat. If more than two people regularly use the room, add 600 BTU/h for each extra person.
- Kitchen: cooking adds a lot of heat — add 4,000 BTU/h.
- Tall ceilings: the chart assumes 8 ft ceilings. With higher ceilings there's more air to cool; size up.
Why an oversized air conditioner is a problem
It's tempting to buy the biggest unit you can afford. But an air conditioner does two jobs: it lowers the temperature and removes moisture. An oversized unit cools the room so fast that it switches off before it has run long enough to dry the air. The result is a room that feels cold and clammy, with more on–off cycling that wastes energy and wears the compressor. An undersized unit has the opposite problem: it runs constantly and can't keep up on the hottest afternoons. The closest size to the calculated capacity is the right one.
Window, portable or mini-split?
- Window units are the cheapest per BTU and quite efficient, but block part of a window.
- Portable units are easy to move but less efficient. Compare their SACC (seasonally adjusted cooling capacity) rather than the headline BTU number — it's typically lower.
- Ductless mini-splits are the most efficient and quiet, and heat pump versions also heat. They're a larger investment and usually professionally installed.
Whole-house systems need a load calculation
Rules of thumb are fine for a single room. Central air conditioners and heat pumps should be sized with a proper load calculation — the industry standard is ACCA Manual J — that accounts for insulation, windows, air leakage, orientation and your climate. A house with a well-insulated attic can need a much smaller system than square footage suggests; see the insulation calculator.
Frequently asked questions
How many square feet does 8,000 BTU cool?
About 300 to 350 square feet with 8 ft ceilings, according to the ENERGY STAR chart.
How many BTU for a 500 sq ft room?
About 12,000 BTU/h — the 450 to 550 sq ft band — before adjustments for sun, people or a kitchen.
Does a room air conditioner need a special outlet?
Small units plug into a standard outlet, but larger ones may need a dedicated circuit or a 240-volt outlet. Check the unit's electrical requirements before you buy.
Sources
- ENERGY STAR, Room air conditioners — sizing chart and adjustments.