BTU & Room AC Size Calculator

Find the right cooling capacity for a single room using the ENERGY STAR sizing method, then estimate what it will cost to run.

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Room air conditioner size and running cost

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How it works

Estimate only. Check product labels, your supplier and local rules before you buy or build.

How the AC size calculator works

For a single-room window, wall or portable air conditioner, the simplest method with an official source is the ENERGY STAR sizing chart. It matches floor area to cooling capacity in BTU per hour, assuming 8 ft ceilings, and then applies four adjustments: subtract 10% for a heavily shaded room, add 10% for a very sunny room, add 600 BTU/h for each regular occupant beyond two, and add 4,000 BTU/h for a kitchen. ENERGY STAR then suggests rounding the total down to the nearest thousand when shopping. This calculator applies exactly those rules and shows each step.

Bigger is not better. ENERGY STAR points out that an oversized unit cools the room before it has time to remove humidity, which leaves the air cool but clammy. That is the main reason to size carefully rather than buy the largest unit you can afford.

ENERGY STAR room AC chart

Area to be cooled (ft²)	Approx. m²	Capacity (BTU/h)
100 to 150	9–14	5,000
150 to 250	14–23	6,000
250 to 300	23–28	7,000
300 to 350	28–33	8,000
350 to 400	33–37	9,000
400 to 450	37–42	10,000
450 to 550	42–51	12,000
550 to 700	51–65	14,000
700 to 1,000	65–93	18,000
1,000 to 1,200	93–111	21,000
1,200 to 1,400	111–130	23,000
1,400 to 1,500	130–139	24,000
1,500 to 2,000	139–186	30,000
2,000 to 2,500	186–232	34,000

Square-metre column converted by Vroxify (1 m² = 10.764 ft²). To convert capacity to kilowatts: 1 kW ≈ 3,412 BTU/h.

Running-cost formula

Electrical power (W) = cooling capacity (BTU/h) ÷ EER
Energy per day (kWh) = W × hours ÷ 1000
Cost = kWh × price per kWh

EER (or CEER, the version on US room-AC labels that also counts standby power) is cooling output in BTU/h per watt of electricity. The result is an upper bound for the hours you enter, because in mild weather the compressor cycles off. If your unit lists its power input in watts, you can skip the EER step and use that figure directly. The default price is the US average residential rate for August 2026 (18.16 cents per kWh, U.S. EIA). In the UK, use the unit rate on your bill or the current Ofgem price cap, and in Canada or Australia use your own tariff.

Worked examples

A sunny 15 × 12 ft bedroom used by three people

180 ft² falls in the 150–250 row: 6,000 BTU/h. Very sunny: +10% gives 6,600. One person above two: +600 gives 7,200 BTU/h. Rounding down to the nearest thousand, as ENERGY STAR suggests, points to a 7,000 BTU/h unit.

Running that unit

A 7,000 BTU/h unit with an EER of 11 draws about 7,000 ÷ 11 = 636 W while the compressor runs. Eight hours a day uses 5.1 kWh; at $0.18/kWh that’s about $0.92 a day, or $27.50 over 30 days at most.

A 4 × 5 m room (metric)

20 m² is 215 ft², which falls in the 6,000 BTU/h row (about 1.8 kW of cooling).

Limits of this method

Room units only. The chart is for single-room air conditioners. Central air, heat pumps and multi-room mini-split systems should be sized with a proper load calculation. In the US that is ACCA Manual J, done by a qualified HVAC contractor.
Ceilings. The chart assumes 8 ft (2.4 m) ceilings. Taller rooms hold more air, and ENERGY STAR notes they may need more capacity.
Climate and insulation are not inputs here. Very hot climates, poor insulation and big west-facing windows push the need up; the sunny adjustment captures only part of that.
Electrical supply. Larger units can need a dedicated circuit or a different outlet. Follow the manufacturer’s installation instructions and use a licensed electrician for any wiring. This site doesn’t give electrical advice.

Frequently asked questions

What size air conditioner do I need for a 12 × 12 room?

A 12 × 12 ft room is 144 ft², which falls in the ENERGY STAR chart’s 100–150 ft² row: 5,000 BTU/h. Adjust for sun, extra people or a kitchen.

How many BTU do I need per square foot?

There’s no single number per square foot. ENERGY STAR uses a chart instead, for example 6,000 BTU/h for 150–250 ft² and 12,000 BTU/h for 450–550 ft², then adjusts for sun, people and kitchens.

Is a bigger air conditioner better?

No. ENERGY STAR notes that an oversized unit cools the room before it removes the humidity, which leaves the room feeling damp and clammy and wastes energy.

How much does it cost to run a window AC?

Divide the BTU/h by the EER to get watts, multiply by the hours run, and divide by 1,000 to get kWh, then multiply by your price per kWh. A 7,000 BTU/h unit with an EER of 11 running 8 hours a day at $0.18/kWh costs at most about $27.50 a month.

Can I use this for central air or a mini-split?

Not for a whole house or several rooms. Those systems need a room-by-room load calculation, such as ACCA Manual J in the US, done by a qualified contractor.

Sources

Last reviewed 2026-10-27. Found a mistake? Tell us and we will fix it.