Window AC Size Calculator

How window AC sizing works

Air conditioners are sold by cooling capacity in BTU per hour, and buying the wrong number is the most common mistake people make. Too small and the room never gets comfortable on the hottest afternoon. Too big and the room gets cold and damp, because the unit satisfies the thermostat before it has run long enough to pull moisture out of the air.

The sizing method starts with floor area. ENERGY STAR's guide to choosing the right sized window AC gives a simple chart in 100-square-foot steps, and this calculator uses that chart as its base. Then it applies four adjustments:

• Sun exposure. Add 10 percent for a room that gets direct sun much of the day, and subtract 10 percent for a room that is heavily shaded.
• Occupants. The chart assumes two people. Add 600 BTU for each additional person who regularly uses the room.
• Kitchen. Add 4,000 BTU if the unit cools a kitchen, because cooking appliances pour heat into the room.
• Ceiling height. The chart assumes an 8-foot ceiling, so a taller room is scaled up in proportion to its extra volume.

The Department of Energy's room air conditioner guidance covers the efficiency side, and it makes the same point about sizing: an oversized unit is not a better unit.

Worked example: a sunny 400 square foot living room

A 20 by 20 foot living room with an 8-foot ceiling, big west-facing windows and three people who use it every evening.

• Floor area: 20 x 20 = 400 sq ft
• Base from the chart, 350 to 450 sq ft: 10,000 BTU
• Sunny adjustment, +10 percent: 11,000 BTU
• Three people, so 600 BTU for the third: 11,600 BTU
• Not a kitchen: no change
• Round up to the next standard size: 12,000 BTU

At 12,000 BTU the unit draws roughly 1,043 watts while the compressor runs. At 18 cents per kWh that is $0.19 an hour at full draw, and about $34 a month at 8 hours a day once you account for the compressor cycling off roughly a quarter of the time.

Note what the rounding did. The calculated load was 11,600 BTU, and there is no 11,600 BTU window unit, so you buy the 12,000. That small oversize is normal and harmless. Buying the 15,000 BTU unit instead would be a real oversize, and you would feel it as a clammy room.

Worked example: a shaded 200 square foot bedroom

A 16 by 12.5 foot bedroom with one north-facing window, heavily shaded by trees, used by two people.

• Floor area: 16 x 12.5 = 200 sq ft
• Base from the chart, 150 to 250 sq ft: 6,000 BTU
• Heavily shaded, -10 percent: 5,400 BTU
• Two people: no adjustment
• Round up to the next standard size: 6,000 BTU

At 6,000 BTU the unit draws about 522 watts and costs $0.09 an hour at 18 cents per kWh, or about $17 a month at 8 hours a night. A 5,000 BTU unit would also work here and costs slightly less to buy, but it would run flat out on the hottest nights and would be marginal if the room were a little bigger.

Bedrooms are the easiest rooms to size, because the load is stable and there is no cooking or big solar gain. The only adjustment people forget is the person count. Three people sleeping in a small room means 600 BTU more than the chart suggests.

BTU sizing chart and the four adjustments

Start with the area row, then apply the adjustments. This is the chart ENERGY STAR publishes:

Then adjust:

Anything above 1,000 square feet is outside what a window unit handles well. A 1,200 square foot open plan space works out to 26,400 BTU, and at that point you are looking at two units, a portable floor model, or a ducted system.

What it costs to run, by unit size

The purchase price is the smaller number over the life of a window air conditioner. Here is what each size costs to run at 8 hours a day, 18 cents per kWh, and a compressor duty cycle of about 75 percent:

A summer is counted as three months at 8 hours a day. Your electricity rate matters as much as the unit size: at 12 cents per kWh, which is the low end of US residential rates, a 12,000 BTU unit runs about $22 a month instead of $34. The EIA's monthly electricity data shows the state-by-state spread, and it runs from under 11 cents to over 40 cents per kWh.

The watts figure is estimated at 11.5 BTU per watt, which is a reasonable efficiency for a modern window unit. An older or cheaper unit may draw 20 percent more for the same cooling, which is why the ENERGY STAR label is worth checking before you buy.

Why an oversized unit is a mistake

It feels wrong that a bigger air conditioner could be worse, but the physics is straightforward. An air conditioner does two jobs: it lowers the temperature, and it removes humidity by condensing water out of the air onto the cold coil. The second job needs run time.

An oversized unit drops the room temperature so fast that the thermostat shuts it off in a few minutes. The coil never stays cold long enough to drain much water, so the room ends up cold and clammy at the same time. That is the feeling people describe as a room that never dries out.

There are three more costs to oversizing:

• More money up front. A 15,000 BTU unit costs $80 to $150 more than the 12,000 BTU unit the room needs.
• Shorter compressor life. Frequent on and off cycling is harder on a compressor than running steadily.
• Noisier and less comfortable. The unit blasts cold air in short bursts instead of holding a steady temperature.

If you are between two sizes, go with the smaller one for a shaded bedroom and the larger one for a sunny room with a west-facing window or a kitchen. And if the room already has a unit that is too big, run it on a higher fan setting so the air keeps moving after the compressor stops, which spreads the cold air and gives the coil more time to dry the room out.