What size air conditioner do I need?

Nearly every sizing page on the internet gives you “20 BTU per square foot” and stops. That rule is close enough in the middle of the range and meaningfully wrong at both ends, so we are going to use the actual table instead — and then do the two steps that the rule of thumb skips entirely.

Length times width, in feet. That is it. A 12 by 14 bedroom is 168 square feet.

Two things worth getting right while you have the tape out. If the space is open-plan — a living room that flows into a kitchen with no door between them — you are cooling all of it, so measure all of it. A room unit cannot cool one half of a shared volume. And if your ceilings are higher than eight feet, note that, because the table assumes eight.

This is the ENERGY STAR room air conditioner sizing table, reproduced exactly. It is the closest thing to an official answer that exists for consumer room units.

Our 168 sq ft bedroom falls in the 150–250 band: 6,000 BTU.

Notice the table is not linear. At 150 sq ft you need about 33 BTU per square foot; at 2,500 sq ft, about 14. Small rooms need proportionally more because the walls, ceiling and windows — where the heat actually comes in — do not shrink as fast as the floor area does. This is exactly where the 20-BTU rule of thumb misleads you, and small rooms are the common case.

ENERGY STAR publishes four corrections. All four are quoted verbatim below, and they matter — a sunny kitchen can shift the answer by a whole capacity band.

Worked through on our 6,000 BTU bedroom: south-facing with afternoon sun, so add 10% → 6,600 BTU. Two people sleep there, which is the table’s baseline, so no change. Not a kitchen. Round to the nearest available capacity and you are buying a 7,000 BTU unit, or an 8,000 if 7,000 is hard to find in the form factor you need.

The kitchen adjustment is the one people forget and it is the biggest: +4,000 BTU. An oven and a cooktop are space heaters that happen to make dinner. A 250 sq ft open-plan kitchen-diner needs 7,000 + 4,000 = 11,000 BTU, not 7,000.

Higher ceilings: the table assumes eight feet, so a 10-foot ceiling is 25% more air to move. ENERGY STAR’s own guidance is to select a unit with a higher airflow rating rather than simply buying more capacity, because the extra volume is a circulation problem more than a heat-load one.

This is the step almost nobody takes, and skipping it is how people end up with a “14,000 BTU” machine that cannot hold a bedroom.

A portable air conditioner’s box figure is measured under a standard that ignores the heat its exhaust duct radiates back into your room and the hot outside air that infiltrates to replace what it blew out. The DOE-rated SACC figure subtracts both.

Real published pairs, from the manufacturers themselves: the Midea Duo MAP14S1TBL is 14,000 BTU on the box and 12,000 BTU SACC. The Whynter ARC-14S is also 14,000 on the box and 9,500 BTU SACC — a 32% reduction.

So take the SACC number to the table above. 9,500 BTU SACC is a 350–400 sq ft machine, not a 550 sq ft one. The full explanation of why the two numbers differ is here, and it is worth ten minutes before you spend several hundred dollars.

Window units need no such correction. Their labeled capacity is their capacity, which is one of several reasons a window unit wins whenever one will physically fit.

The studio is the instructive one. On floor area alone it wants 10,000 BTU. The kitchenette adds 4,000 and pushes it to a completely different class of machine. Anyone sizing that room off a square-footage calculator alone buys 4,000 BTU too little and never works out why the place is warm when they cook.

Sizing is only the first constraint. The second is what will physically go in your window, and the third is what it costs to run.

• Window units by capacity — ranked on certified CEER and annual kWh, with real running costs.
• Portable units — for when a window unit will not fit, sized on SACC.
• What renters can install — the constraint that usually decides the form factor.
• The full BTU chart — if you just want the table on its own.

And if your room turns out to need less than 5,000 BTU, or you cannot install anything at all, a fan and a blind may genuinely be the right answer. We would rather say that than sell you a compressor.