How to size an air conditioner for a room
6 min read · Updated 2026-10-12 · BuildCalc
Sizing an air conditioner looks like a simple matter of matching a number to a room, and it is — but in the right direction. Too small a unit runs continuously and never quite reaches its set point; too large a unit cools the air in short bursts, leaving the room clammy, cycling the compressor and using more electricity than a correctly sized machine. Bigger is not better.
The rule of thumb, and its adjustments
A reasonable starting point is around twenty BTU per square foot of floor area in a normal room, adjusted upward for taller ceilings because you are cooling a larger volume of air. A room in full sun all day needs roughly ten per cent more, a shaded one a little less.
Then add for heat sources. Each occupant beyond the first adds about four hundred BTU, and a kitchen in the same space adds several thousand more because cooking produces heat directly. Computers, televisions and large west-facing windows all contribute as well, though the simple method does not capture them.
From BTU to tons and unit types
Twelve thousand BTU makes one ton of cooling, which is how larger systems are often described. Round the calculated figure to the nearest sensible unit size: about five to six thousand BTU is a small portable unit, nine to twelve thousand a large window unit, twelve to twenty-four thousand a mini-split, and beyond that a multi-zone or central system.
Matching the type to the space matters as much as the capacity. A window or portable unit is cheap and simple but noisy and inefficient at scale. A mini-split is quieter, more efficient and can heat as well as cool. A central system makes sense where several rooms need conditioning at once and ductwork exists or can be installed.
Why a professional load calculation exists
The rule of thumb ignores insulation values, glass area, orientation, air leakage and local design temperatures, all of which can move the answer substantially. A formal load calculation, often called a Manual J in the United States, accounts for all of them and is what an installer should produce before quoting.
Use the estimate to shortlist and to sanity-check quotes. If an installer recommends a system twice the size your own figures suggest, ask why, and expect an answer about windows, insulation or the condition of an adjacent space. If there is no answer, get a second quote.
Run your own numbers
Free calculators that answer the “how much” version of this question. They run entirely in your browser — nothing you type is uploaded or stored.
Common questions
How many BTU do I need per square foot?
About twenty for a normal room with a standard ceiling, adjusted up for extra height, full sun and additional occupants.
Is a bigger AC unit better?
No. Oversized units cycle on and off, leave the room humid and wear out faster. Size the unit to the actual load.
BuildCalc provides general information and planning estimates only, not professional construction, engineering or electrical advice. Material quantities are illustrative and depend on your own project, products and local codes. See our full disclaimer.