What Size Air Conditioner Do I Need? BTU by Room Size

Every July somebody buys a 12,000 BTU window unit for a 180 sq ft bedroom because the 6,000 looked too small sitting on the shelf next to it. Two weeks later the room is cold and strangely damp, the compressor kicks on and off every few minutes, and the fix is a smaller air conditioner — which is not what anyone wants to hear after wrestling one into a window frame.

AC sizing is a table lookup, not a judgment call. The Energy Star square-footage chart, the same one printed on the back of most window-unit boxes, turns floor area into a base BTU figure. Everything after that is correcting what the table quietly assumes: an 8 ft ceiling, average sun, two occupants, no kitchen. Your room almost certainly breaks one of those. The job is to find the base, apply the corrections, and then resist the instinct to round up "to be safe."

The formula

Four lines, in order:

• base BTU = Energy Star table value for length × width
• BTU = base × (ceiling height ÷ 8) × sun factor
• + (occupants − 2) × 600 — only counts above two people
• + 4,000 if the kitchen is inside the cooled space

Round the result to the nearest 100, then buy the next common unit size at or above it: 5,000 · 6,000 · 8,000 · 10,000 · 12,000 · 14,000 · 18,000 · 21,000 · 24,000 · 28,000 · 34,000.

Measure length and width in feet and let the area fall out. One thing to get right before you touch the table: open layouts are one zone, not one room. A living room that flows into a dining area is a single space the unit has to serve, and sizing for the half the unit sits in guarantees disappointment. Measure the whole connected footprint — the same square footage you would tally up if you were pricing flooring for it.

What the table assumes away

The chart is built for an average room. Four inputs move you off that average, and they are the difference between a unit that keeps up on a 95° day and one that runs flat out and loses.

Ceiling height is the most-skipped adjustment, and it is pure volume. BTUs cool air, not floor tiles, so a 10 ft ceiling holds 25% more air than the 8 ft the table assumes. The math scales by height ÷ 8, which means a lofted room jumps a full unit size or two.

Sun exposure swings the number by ±10%. Pick very sunny (× 1.1) for west- or south-facing rooms with big unshaded windows, shaded (× 0.9) for north-facing rooms or heavy tree cover, average (× 1.0) for everything else.

People add about 600 BTU each in body heat — but only past the first two, who are already in the base figure. Use your regular occupancy, not the one dinner party a year.

A kitchen in the cooled space adds a flat 4,000 BTU. A running range and a refrigerator dumping condenser heat into the room is not a rounding error; in a normal-sized room it is the largest correction on the list.

A worked example

Take a 20 × 20 ft family room with strong afternoon sun, standard 8 ft ceilings, two people, no kitchen:

• Area: 20 × 20 = 400 sq ft
• Table base for 400 sq ft: 10,000 BTU
• Ceiling: 8 ÷ 8 = × 1 → still 10,000
• Sun: very sunny, × 1.1 → 11,000 BTU
• People and kitchen: + 0
• Needed: 11,000 BTU → the next unit on the ladder is 12,000

Now the small end. A 15 × 12 ft bedroom is 180 sq ft, which falls in the table's 150–250 sq ft band: 6,000 BTU. Standard ceiling, average sun, two sleepers, no kitchen — nothing to adjust. You need 6,000 and you buy exactly 6,000. That is a real result, not a suspiciously round one, and it is the size most people talk themselves out of.

Bigger is not safer

Oversizing is the window-unit mistake, and it makes rooms feel worse rather than better.

An oversized unit blasts the room to temperature in a few minutes and shuts off before it has run long enough to condense any moisture out of the air. That is short cycling. The result is a room that is cold and clammy at the same time: cave-damp, sticky, and somehow still unpleasant at 72°. You also pay for it twice, in start-up surges and in compressor wear.

A correctly sized unit runs longer, steadier cycles, and it dehumidifies the whole time it cools. Comfort is temperature and humidity, and only run time buys you the second one.

So when you land between sizes, take the next size up only if the room has heat sources the calculator did not capture: a wall of computers, an uninsulated attic overhead, a stove just outside the doorway. Otherwise trust the number.

Portable units need a skeptical eye

The BTU number on a portable AC and the BTU number on a window unit are not measuring the same thing. Portables lose a large chunk of their capacity through the exhaust-hose setup, so a box marked "12,000 BTU" often delivers real-world cooling closer to a 7,000–8,000 BTU window unit.

The honest figure is the SACC (seasonally adjusted cooling capacity), required on US portable labels since 2020. Compare SACC against your calculated number, never the headline BTU. If you need 11,000 BTU, you need 11,000 SACC.

A window unit of the same rated size out-cools a portable every time. Go portable only when a window unit genuinely cannot be installed.

CEER is a separate decision

BTU says how much a unit cools. CEER says how much that cooling costs you. They are unrelated, and people conflate them constantly.

CEER (combined energy efficiency ratio) is the number on the yellow EnergyGuide label, and it includes standby power. For window units, 11 is acceptable, 12+ is good, and Energy Star–certified models sit around 12.1 or better. Between two 8,000 BTU units, both cool identically; the higher-CEER one just does it on fewer watts, which is the entire difference in your summer bill. Roughly 8–10% less energy per point of CEER.

Size first, then shop CEER within that size. Never trade up a size to get a better efficiency number.

When you are past window-unit territory

Above 34,000 BTU, the ladder runs out and you are no longer shopping for a window unit at all. That is central-air or multi-zone mini-split scale. Whole-floor open plans, big lofted great rooms, and kitchen-plus-living combinations get there faster than people expect, mostly because the kitchen's 4,000 and a tall ceiling's × 1.25 stack on top of an already large floor area.

Common mistakes to avoid

• Sizing up "to be safe" — the fastest route to a cold, clammy, short-cycling room.
• Measuring only the room the unit sits in on an open plan, instead of the whole zone it has to cool.
• Ignoring ceiling height — a 10 ft ceiling is 25% more air and usually a full size jump.
• Comparing a portable's headline BTU to a window unit's instead of its SACC.
• Forgetting the kitchen toggle — 4,000 BTU moves most rooms up a size or two.
• Shopping CEER before BTU — efficiency never fixes wrong capacity.

Measure the whole cooled space, look up the base, apply the four corrections, and buy the smallest unit that clears the number. It will run longer than the oversized one on the shelf next to it, and the room will feel dry instead of merely cold. While you have the tape out and the room measured, it is a good moment to price the paint or the drywall you have been putting off; the same wall and floor measurements feed all three.