What Size Air Conditioner Do I Need?

Bigger is not better, and this is the whole point

An oversized air conditioner cools the air to the thermostat setting quickly and then shuts off.

That sounds efficient. It isn't — because removing humidity takes running time. A unit that short-cycles never runs long enough to pull moisture out of the air.

The result is a room that's cold and clammy rather than comfortable, plus extra compressor wear from constant starting and stopping.

An undersized unit runs forever and never quite gets there. Both are unpleasant — but only one is the mistake people actually make. Almost nobody buys an air conditioner too small.

The rule, and the adjustments that matter

20 BTU per hour per square foot. Published guidance runs 20–25; 20 is the starting point.

A 15 × 20 room is 300 sq ft → 6,000 BTU.

Then ENERGY STAR's own adjustments, which are unusually specific for a rule of thumb:

Condition	Adjustment
Heavily shaded	−10%
Very sunny	+10%
Each person beyond two	+600 BTU
Kitchen	+4,000 BTU

That kitchen figure is large and it isn't a typo. Cooking puts a great deal of heat into a room — on our 300 sq ft example it adds 4,000 to a 6,000 BTU base, a two-thirds increase before any other adjustment. A unit sized on floor area alone will simply never keep up in a kitchen.

Sun exposure is the adjustment worth thinking hardest about, because it's the one you judge rather than measure. Which way does the room face, and what's outside the window? A west-facing room with unshaded glass takes the full afternoon sun at exactly the hour you most want it cool. A north-facing room behind a mature tree is a different problem entirely.

The tool defaults to very sunny, on the reasoning that the sunny room is usually the one you're trying to cool in the first place. Set it to average or shaded if that isn't yours — it's a 20% swing between the extremes, which on a 300 sq ft room is 1,200 BTU and easily a whole unit size.

The rule ignores the third dimension

"20 per square foot" quietly assumes a ceiling of about 8 feet. But an air conditioner cools air, not floor.

A 150 sq ft room with a 12-foot ceiling holds half as much air again as the same room with an 8-foot one — so the basic figure under-sizes it by 50%: 3,000 BTU becomes 4,500.

The calculator scales by the height ratio and shows it as a ceiling height factor — 1.00 at a standard 8 ft, 1.50 at 12 ft. It shows room volume too, because that's what the unit is actually working on.

When your answer lands between two sizes

It usually will. Units come in fixed steps — 5,000, 6,000, 8,000, 10,000, 12,000 BTU.

Take the nearer size, not automatically the bigger one. Prefer the smaller of the two when the room is well insulated or well shaded.

Rounding up feels like buying headroom. It's buying the short-cycling problem.

This sizes a room unit, not a system

Worth being clear about.

Properly sizing a central or whole-house system means a Manual J load calculation — insulation levels, window area and orientation, air leakage, local climate design temperatures, and a good deal more.

This is the room-unit rule of thumb. Genuinely adequate for choosing a window or portable unit for one room; not adequate for specifying a system.

If someone quotes you a central system without doing a load calculation, that's worth noticing.

What the rule can't see

Three things it's worth knowing are missing, because each can matter more than the adjustments:

Insulation and air leakage. A poorly insulated room with a draughty window loses cooled air continuously, and no amount of BTU fixes a hole. Sealing is usually cheaper than upsizing.

Window area. Glass is where most solar heat arrives, and two rooms of identical floor area with very different glazing are not the same cooling problem. The sun adjustment is a crude proxy for this.

What's above you. A top-floor room under a hot roof behaves quite differently from the same room with another storey over it.

The rule of thumb absorbs all of this into "average conditions" — which is exactly what a load calculation exists to stop doing.

Efficiency is a separate question

Size decides whether the room is comfortable. Efficiency decides what it costs to run.

Look for an ENERGY STAR label and an EER of 10 or above.

And note the order: a correctly sized efficient unit beats an oversized efficient one on both counts. An efficiency rating cannot rescue a unit that's the wrong size for the room.

Print the sizing sheet — it has the adjustments table and a room-by-room grid, plus the practical checks (window fitting, circuit, condensate) that decide whether the unit you chose can actually go in.

Sources: ENERGY STAR sizing guidance, via Consumer Reports · PickHVAC — BTU to room size