Window ACBTU Lab

How many BTU does your window AC need?

Room size, sun, people and kitchen heat set the BTU. Your window and wall outlet decide which unit you can actually install. This calculator checks all of it at once.

Room
Standard is 8 ft (2.4 m)
+600 BTU per person above 2
Window
Height of the opening with sash fully open
Running cost
ENERGY STAR ≈ 12+, inverter 15+
Recommended size–
Power / outlet–
Window fit–
Cost per season–
Inverter (CEER 15) saves–per season vs. your rating

The window AC sizing chart

The US ENERGY STAR program publishes a simple chart that matches room area to air conditioner capacity in British Thermal Units per hour (BTU/h). It assumes an 8-foot ceiling, average insulation and two occupants. This calculator starts from that chart and then applies the official adjustments: reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny room, add 600 BTU for each regular occupant beyond two, and add 4,000 BTU if the unit is used in a kitchen. We also scale for ceiling height, top-floor rooms and hot climates, which the basic chart does not cover.

Area to be cooled (sq ft)Capacity needed (BTU/h)
100 – 1505,000
150 – 2506,000
250 – 3007,000
300 – 3508,000
350 – 4009,000
400 – 45010,000
450 – 55012,000
550 – 70014,000
700 – 1,00018,000
1,000 – 1,20021,000
1,200 – 1,40023,000
1,400 – 1,50024,000

Why bigger isn’t better

An oversized window unit cools the air quickly and then shuts off before it has removed much moisture. The room feels cold and damp, and the compressor cycles on and off more often, which wastes energy and shortens its life. An undersized unit runs non-stop on hot afternoons and may never reach the set temperature. Picking the size closest to your calculated load gives the best comfort and the lowest bills.

Will it fit your window?

Most window air conditioners are designed for double-hung windows, where the lower sash slides up. Every model lists a minimum and maximum window width (side panels or accordion curtains fill the gap) and a minimum opening height. Small 5,000–6,000 BTU units usually fit openings about 23–36 inches wide and 12–13 inches high. Mid-size 8,000–12,000 BTU units need around 14–15 inches of height. Large 15,000–25,000 BTU units are heavier, often need 17–19 inches of height and a width of at least 26–28 inches, and should be installed with a support bracket.

Slider windows need a vertical “slider/casement” air conditioner that is tall and narrow. Casement windows that crank outward cannot hold a standard unit at all; the options are a casement-specific model with a custom panel, a through-the-wall unit or a portable air conditioner.

115-volt or 230-volt?

Units up to about 12,000–14,000 BTU plug into an ordinary 115-volt, 15-amp household outlet. Some 15,000–18,000 BTU models are still 115V but draw 12 amps or more, so they should be on a circuit with nothing else connected. Most units of 18,000 BTU and above need a 208/230-volt outlet with a special plug shape, which normally requires an electrician. Never use an ordinary extension cord with an air conditioner.

What is CEER?

The Combined Energy Efficiency Ratio (CEER) is the official efficiency rating for room air conditioners. It divides cooling output by power use and includes standby power. Higher is better. Basic models score around 10–11, ENERGY STAR models about 12 or more, and inverter window units can exceed 15, using up to 35% less electricity while also running much more quietly.

Window AC FAQ

What size window AC do I need for a 12×12 room?
A 12×12 room is 144 sq ft, so a 5,000 BTU unit is enough in normal conditions. Choose 6,000 BTU if it is very sunny or on a top floor.
How many square feet does a 10,000 BTU window AC cool?
About 400–450 sq ft with standard ceilings and average sun, or slightly less in hot climates.
Can I put a window AC in a sliding window?
Yes, but you need a vertical slider/casement model. A standard horizontal unit will not seal properly in a sideways-sliding window.
Does a window AC need a dedicated circuit?
Small units do not, but anything drawing more than about 7.5 amps should not share a circuit with other heavy loads. Units of 15,000 BTU and above should always have their own circuit.
How much does it cost to run a window AC?
A 10,000 BTU unit at CEER 12 draws about 830 watts. Eight hours a day costs around $1.10 at $0.17/kWh, or about $110 for a 100-day summer.

About Window AC BTU Lab

Window AC BTU Lab is an independent calculator for anyone shopping for a room air conditioner. It brings together the ENERGY STAR sizing chart, typical manufacturer window dimensions and electrical requirements in one place, so you can narrow your choice before you buy. We are not affiliated with any brand or store.