Air Conditioner BTU Calculator — Size by Room, Not Guesswork

Real-world use cases

• Size a window AC for a sunny 250 sqft living room

  A south-facing living room is 250 sqft with a standard 8 ft ceiling and seats four people. Enter 250 ft², 8 ft, orientation "Faces the sun", 4 people. The base is 5000 BTU, the sun adds 500, and two extra people add 1200 — about 6700 BTU, which rounds to a 7000–8000 BTU window unit (just under 1 匹). A flat "20 per sqft" rule would have said 5000 and left you short on the hottest afternoons. The breakdown shows exactly where the extra 1700 BTU came from.

• Pick the right 匹 for a Chinese apartment bedroom

  A 14 m² bedroom, 2.7 m ceiling, north-facing (shaded), two occupants. Switch units to m², enter 14 and 2.7, orientation "Shaded side". The tool resolves the area to about 150 sqft, scales for the 2.7 m ceiling, and lands near 3300 BTU — comfortably a 1 匹 unit. Knowing it is 1 匹 and not 1.5 匹 saves both purchase price and running cost; an oversized 1.5 匹 in a small shaded bedroom would short-cycle and feel clammy.

• Account for kitchen heat before buying

  A 120 sqft open kitchen with a 9 ft ceiling and the stove running during dinner. Enter 120 ft², 9 ft, and tick "It is a kitchen". Base 2400, ceiling adds about 300, kitchen adds a flat 4000 — roughly 6700 BTU, far above the 2400 a floor-area rule alone would suggest. The kitchen line in the breakdown makes it obvious why a kitchen needs a bigger unit than a same-size bedroom.

• Compare a loft vs a standard room of the same footprint

  Two 200 sqft rooms, one with an 8 ft ceiling and one a 12 ft loft. Run both: the standard room lands at 4000 BTU, the loft at 6000 BTU — a 50% jump purely from the extra air volume. Seeing the two side by side explains why the loft conversion that "should" use the same AC as the bedroom below it never quite cools down on a hot day.

• Decide between a split unit and a portable

  You have a 180 sqft home office, west-facing glass, three people on call days. The tool gives about 4500 BTU. Portable units lose 20–30% of their nameplate to the exhaust hose heat, so a 5000 BTU portable effectively delivers ~3700 — under your need. The number tells you to either step the portable up to 8000 BTU or fit a properly-sized split, rather than buying a portable that will run flat-out and still lose.

Common pitfalls

• Sizing on floor area alone and ignoring ceiling height. A flat "20 BTU per sqft" assumes an 8 ft ceiling; a 12 ft loft of the same footprint needs about 50% more capacity. Always enter the real ceiling height — this is the most common reason a "correctly sized" unit cannot keep up.

• Assuming bigger is safer and jumping two sizes up. An oversized AC short-cycles — it cools the air fast, shuts off before it dehumidifies, and leaves the room cold but clammy while wearing the compressor and burning startup current. Pick the next standard size at or just above the calculated load, not two sizes up.

• Forgetting kitchen and occupancy heat. The stove in a kitchen adds about 4000 BTU and each person past two adds 600 BTU/h. A 120 sqft kitchen full of people during dinner has a very different load from an empty 120 sqft bedroom — tick the kitchen box and set the real headcount.

Privacy