BTUFit › Window AC size
Window AC BTU calculator
Window units run from 5,000 to 24,000 BTU, and the price steps are steep. This tells you the smallest unit that will actually hold your room, straight from the ENERGY STAR method.
Window AC size chart
| Room area | Load (BTU/h) | Unit size |
|---|---|---|
| 100 – 150 sq ft | 5,000 | 5,000 BTU |
| 150 – 250 sq ft | 6,000 | 6,000 BTU |
| 250 – 300 sq ft | 7,000 | 8,000 BTU |
| 300 – 350 sq ft | 8,000 | 8,000 BTU |
| 350 – 400 sq ft | 9,000 | 10,000 BTU |
| 400 – 450 sq ft | 10,000 | 10,000 BTU |
| 450 – 550 sq ft | 12,000 | 12,000 BTU |
| 550 – 700 sq ft | 14,000 | 14,000 BTU |
| 700 – 1,000 sq ft | 18,000 | 18,000 BTU |
Assumes 8 ft ceilings, average sun, two people. Sunny room: +10%. Shaded: −10%. Kitchen: +4,000 BTU. Beyond ~1,000 sq ft you're into the largest window units; compare a mini split before buying a 230V window unit.
Before you buy: the window itself
- Measure the opening. Units list minimum and maximum window widths; double-hung windows are the standard case, sliders and casements need special units or kits.
- Check the circuit. 5,000–12,000 BTU is fine on a normal outlet; 15,000 on a lightly loaded circuit; 18,000+ usually means 230V.
- Mind the tilt. A slight backward tilt drains condensate outside instead of onto your floor; most "my new AC leaks" complaints are installation, not the unit.
Frequently asked questions
What size window AC for a 10×10 or 12×12 bedroom?
A 10×10 room (100 sq ft) sits at the bottom bracket: 5,000 BTU, the cheapest unit in the store is genuinely the right one. A 12×12 room (144 sq ft) is also 5,000 BTU territory unless it's very sunny, in which case step to 6,000.
Do window ACs need a special outlet?
Up to about 15,000 BTU they run on a normal 115V household circuit (check the amps; big 115V units want a lightly loaded 15A or a 20A circuit). At 18,000–24,000 BTU most models are 230V, which means an electrician unless you happen to have the outlet. That electrical line is exactly where a mini split starts making more sense.
Does it matter which way my window faces?
The window itself, not much, but the room's sun load does. West- and south-facing rooms with big glass run hot in the afternoon; that's the +10% "very sunny" adjustment. A north-facing shaded room can take the −10%.
Window AC or portable AC?
A window unit, if the window allows it. Portables give up 20–30% of their rated capacity to duct losses (that's why the honest SACC rating is lower than the marketing BTU number) and cost more per delivered BTU. Buy a portable for rooms where a window unit is impossible or prohibited.