Sizing a room air conditioner by hand: the 20 BTU/ft² rule
Air conditioner listings all lead with a BTU number — 8,000, 10,000, 12,000 — with no explanation of what it means for your room. The sizing rule behind most buying guides is simple enough to do on paper: about 20 BTU of cooling capacity per square foot of floor area.
The method
- Measure the room's floor area in square feet (length × width).
- Multiply by 20. That's the target BTU rating to shop for.
- Adjust up from there for anything that adds heat the base rule doesn't know about: direct sun, a kitchen, a high ceiling, or more than two regular occupants.
In one line: target BTU = floor area (ft²) × 20.
A worked example
A bedroom measures 12 ft by 12.5 ft.
- Floor area:
12 × 12.5 = 150 ft². - Target BTU:
150 × 20 = 3,000 BTU.
That lands below most window units' smallest listed size (usually 5,000–6,000 BTU), which is normal for a small bedroom — the rule tells you not to overshoot into an 8,000 BTU unit that will short-cycle and leave the room clammy rather than dry.
A second example, with adjustments
A living room is 18 ft by 20 ft, has a west-facing window that gets afternoon sun, and regularly holds four people watching a movie.
- Floor area:
18 × 20 = 360 ft². - Base target:
360 × 20 = 7,200 BTU. - Sunny-room guides commonly add about 10% for direct afternoon sun:
7,200 × 1.10 = 7,920 BTU. - Extra occupants beyond two are often costed at roughly 600 BTU each; two extra people adds
2 × 600 = 1,200 BTU. - Adjusted target:
7,920 + 1,200 = 9,120 BTU— enough to round up to a common 9,500–10,000 BTU unit rather than buy right at the base 7,200 BTU figure.
Why bigger isn't automatically better
An oversized unit cools the air fast, satisfies the thermostat, and shuts off before it has run long enough to pull much humidity out of the room — the room ends up cold and damp rather than comfortable. The 20 BTU/ft² rule is deliberately a starting point, not a ceiling: it is meant to be nudged up a little for real heat sources, not doubled "to be safe."
What the rule doesn't know
It has no idea about ceiling height above the standard 8 feet, how many exterior walls the room has, local climate, or whether the room is over an unheated garage. Manuals and manufacturer sizing charts that ask for those details exist for a reason — this rule is for getting into the right range before you compare specific models, not for replacing a proper load calculation on an unusual room.
The free Room AC BTU calculator runs this same multiplication once you enter a floor area, so you can try a few room sizes side by side.