Size a window AC by square feet, then adjust for sun and shade

Multiply the room's length by its width in feet and read the capacity off ENERGY STAR's room air conditioner sizing chart: 150 up to 250 square feet needs 6,000 BTU/h, and 300 up to 350 square feet needs 8,000 BTU/h, on an 8-foot ceiling. Then adjust, as the same page lists: take 10% off for a heavily shaded room and add 10% for a very sunny one. Skip the biggest unit on the shelf; ENERGY STAR says an oversized one costs more to purchase, wastes energy and does not provide better cooling. Measuring is free, the unit is a small buy for renters and owners alike, and a renter checks the lease or building rules first.

Measure the floor in feet, with inches as fractions

Measure the floor of the room you will cool. ENERGY STAR's sizing steps say to measure in feet and turn inches into fractions of a foot: 3 inches is 0.25 ft, 6 inches is 0.5 ft and 9 inches is 0.75 ft, so 12 ft 6 in becomes 12.5 ft. For a square or rectangular room, multiply length by width. For a triangular area, multiply length by width and divide by two. Split an odd-shaped room into squares, rectangles and triangles, work out each, and add them up. A tape measure is all it takes.

Read the capacity off ENERGY STAR's chart

Find the room's area in the left column and read the capacity beside it. The chart assumes an 8-foot ceiling. ENERGY STAR calculates its capacities that way, and for higher ceilings the same page says you may want a unit with a higher CADR. The rows below stop at 1,000 square feet, and the page's chart runs on to 34,000 BTU/h for 2,000 up to 2,500 square feet.

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Take 10% off for shade, add 10% for sun, then add BTU/h for extra people and a kitchen

ENERGY STAR's adjustments come after the chart. If the room is heavily shaded, reduce capacity by 10%. If it is very sunny, increase capacity by 10%. If more than two people regularly occupy the room, add 600 BTU/h for each additional person. If the unit is used in a kitchen, increase capacity by 4,000 BTU/h.

A sunny room also has a free step before any purchase. DOE's Energy Saver guide (2022) says to keep the window coverings closed during the day in summer to block the sun's heat.

Example: a very sunny 175-square-foot bedroom needs 6,600 BTU/h

Example, with published figures and one assumption. Take a bedroom of 12 ft 6 in by 14 ft, shared by two people, that gets direct sun most of the afternoon, so you would call it very sunny; the room, its sun and its two occupants are this example's assumption. With ENERGY STAR's conversion, 12 ft 6 in is 12.5 ft, and 12.5 × 14 = 175 square feet. The same ENERGY STAR chart puts 150 up to 250 square feet at 6,000 BTU/h. Very sunny adds 10%: 6,000 × 0.10 = 600, so 6,000 + 600 = 6,600 BTU/h. With two occupants, the occupant line adds nothing. Compare each unit's rated capacity with 6,600 BTU/h. A 10,000 BTU/h unit, the chart's capacity for 400 up to 450 square feet, is sized for a room more than twice this one's 175.

A bigger unit cools the air but leaves the room clammy

Bigger is not better, ENERGY STAR's buying guidance says: an oversized room air conditioner costs more to purchase, wastes energy and does not provide better cooling. The same page explains the discomfort. A unit that is too large cools the room before it has a chance to remove the humidity, and cool, moist air makes the room feel damp and clammy.

DOE's Energy Saver guide (2022) lists room air conditioners among the options and says an oversized air conditioner performs less efficiently and effectively than a smaller, properly sized unit. To compare two units that both fit the room, turn each one's listed watts into kWh and dollars with the method to price any appliance.

Seal it in, set it level and run it at 78°F

Then seal it in. ENERGY STAR's saving tips say an improperly installed room air conditioner leaks as much air as a 6-square-inch hole. Follow the unit's instructions, use the insulation materials that come with it, and make sure it sits level so the drainage system works. Keep lamps and TVs away from its thermostat, because the extra heat makes it run longer. Set it as high as is comfortable, typically 78°F, the same page says, and run the fan on low on humid days, since slower air removes more moisture. An extra fan spreads the cooled air around, and a ceiling fan saves only when you raise the thermostat while it runs. At the end of the season, remove the unit if you can to minimize heat loss, or put an appropriately sized cover on it for winter.

DOE's 2022 guide adds a monthly job: clean or replace the air conditioner's filter once a month or as recommended.

Before you buy, read the installation section of the unit's manual for its outlet and window requirements. A renter checks the lease or building rules first. If the room has no outlet that matches the plug, or the unit needs a new circuit, that is work for a licensed electrician.