What Size Air Conditioner for 400 Square Feet?

A 400 sq ft room (about 20 × 20 ft) needs about 9,000 BTU of cooling, so look for a 10,000 BTU air conditioner. A very sunny room needs 9,900 BTU, and a kitchen 13,000 BTU.

Change any number below to match your project. The ac btu calculator works for any size.

Room size

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How it's worked out

  1. The ENERGY STAR chart lists 350 to 400 sq ft at 9,000 BTU per hour.
  2. Adjust for the room: +10% for strong sun, −10% for heavy shade, +600 BTU for each regular occupant past two, +4,000 BTU for a kitchen, and scale up for ceilings over 8 ft.
  3. Pick the nearest common unit size at or just above the result.

400 sq ft: size by room conditions

RoomBTU neededUnit to look for
Normal room9,000 BTU10,000 BTU
Very sunny (+10%)9,900 BTU10,000 BTU
Heavily shaded (−10%)8,100 BTU8,000 BTU
4 people use it (+1,200)10,200 BTU10,000 BTU
Kitchen (+4,000)13,000 BTU14,000 BTU
9 ft ceiling (+12.5%)10,100 BTU10,000 BTU

Don't oversize much. A unit that's too big cools the air quickly and shuts off before it pulls out the humidity, so the room ends up cold and clammy.

Other common sizes

More detail: What Size AC Do I Need? BTU and Tons by Square Footage