Air Conditioner BTU: What Size AC You Need
A room air conditioner is sized in BTUs per hour to the floor area it cools; Energy Star's chart runs from 5,000 BTU for 100 square feet to 34,000 for 2,500.
By Liam McCabeContributing writer.
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The short answer
Measure the room, then match its area to a cooling capacity:
- Multiply the room's length by its width, in feet, to get square feet.
- Find that area in the chart below.
- Adjust for sun, shade, extra people or a kitchen.
A bigger unit is not better. Energy Star warns that an oversized air conditioner costs more, wastes energy and can cool the room before it removes the humidity, leaving it feeling damp.
What does BTU mean?
A BTU (British thermal unit) is a unit of heat: about the heat needed to warm one pound of water by one degree Fahrenheit, or roughly 1,055 joules. An air conditioner's capacity is given in BTUs per hour, the amount of heat it can remove from the room each hour. One watt equals about 3.41 BTU per hour, and 12,000 BTU per hour is called one "ton" of cooling.
What size air conditioner do you need?
Energy Star's sizing chart, which assumes an 8 foot ceiling:
| Room area (sq ft) | Capacity (BTU per hour) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
Then adjust:
- Heavy shade: reduce capacity by 10 percent.
- Very sunny room: increase it by 10 percent.
- More than two people use the room regularly: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU.
For an L-shaped or odd room, split it into rectangles, work out each area and add them up.
Window vs portable air conditioner
A window unit sits in the window opening, with its indoor and outdoor sides separated by a partition, so the hot side stays outside. Energy Star also highlights U-shaped and saddle-style window units, which straddle the sill so you can still open and close the window, and keep the noisy compressor outside.
A portable unit rolls on wheels and vents hot air through a hose in a window kit. Many portable units draw room air and blow it outdoors through a single duct, which lowers their overall cooling efficiency. Some collect condensed water in a tank that must be emptied, while others evaporate it out through the hose.
Where the window allows it, a window unit avoids the single-duct efficiency loss. A portable unit needs only a hose kit in the window and can roll from room to room. Of the 89 air conditioners listed on Wiki Shopping on October 26, 2026, 27 are portable, 13 are window units and 25 are split systems. Browse them under home and kitchen.
How we count
Capacities and adjustments are Energy Star's, linked under Sources. As an example of the adjustment, a very sunny 500 square foot room would start at 12,000 BTU and add 10 percent, for about 13,200 BTU; the next listed size up is 14,000 BTU. That example is our own arithmetic. Air conditioner counts by type come from our database on October 26, 2026, and change as listings are added or removed.