What Size Window Air Conditioner Do I Need? BTU Chart

By Ray Bennett · Updated 2 October 2026 · 4 min read · Checked against the sources at the end

What Size Window Air Conditioner Do I Need? BTU Chart

Short answer

Measure the floor area and read ENERGY STAR's chart: 5,000 BTU for 100 to 150 square feet, 6,000 up to 250, 8,000 for 300 to 350, 10,000 for 400 to 450 and 12,000 for 450 to 550. Take off 10 percent for a heavily shaded room, add 10 percent for a very sunny one, 600 BTU for each person past two, and 4,000 in a kitchen. Too big a unit leaves the room damp and clammy.

What size window air conditioner do I need? ENERGY STAR answers it with one chart: measure the floor area of the room, read off the BTU, then adjust for sun, shade, people and cooking. The chart assumes an ordinary 8 foot ceiling.

Getting it right matters in both directions. ENERGY STAR says a unit that is too big cools the room before it removes the humidity, and leaves it damp and clammy. This page walks through the measuring, the chart and the adjustments.

Before you start

What to carry: a tape measure or a laser distance meter, and a pen. Ten minutes.

Gas or electric? Either. A window unit cools one room on electricity, whatever heats the rest of the house.

Measure in feet. ENERGY STAR says to turn inches into fractions of a foot: 12 feet 6 inches is 12.5 feet, 9 inches is 0.75 and 3 inches is 0.25.

What ENERGY STAR is firm about. Too big is not better. A unit that is too large cools the room before it has a chance to remove the humidity, so the room feels damp and clammy.

Fit it properly. ENERGY STAR says a badly installed room air conditioner leaks as much air as a 6 square inch hole. Use the insulation that comes in the box and follow the instructions.

Keep it level. ENERGY STAR says the unit must sit level so its drainage system works.

Measure the room

Check 1Free, five minutes

Work out the square footage

A square or rectangular room. Multiply the length by the width.

A triangular room. Multiply the length by the width and divide by two.

Anything else. ENERGY STAR says to split it into squares, rectangles and triangles, work out each, and add them up.

Two rooms joined by an open doorway. ENERGY STAR’s chart is for the area to be cooled, so measure all the space the unit has to serve, not just the room it sits in.

Read the chart

ENERGY STAR's chart for the common room sizes. The full table, up to 2,500 square feet, is below.

Area to be cooled	Capacity needed
100 up to 150 sq ft	5,000 BTU an hour
150 up to 250 sq ft	6,000
250 up to 300 sq ft	7,000
300 up to 350 sq ft	8,000
350 up to 400 sq ft	9,000
400 up to 450 sq ft	10,000
450 up to 550 sq ft	12,000
550 up to 700 sq ft	14,000
700 up to 1,000 sq ft	18,000
1,000 up to 1,200 sq ft	21,000
1,200 up to 1,400 sq ft	23,000
1,400 up to 1,500 sq ft	24,000
1,500 up to 2,000 sq ft	30,000
2,000 up to 2,500 sq ft	34,000

Adjust for the room

ENERGY STAR's four adjustments, and one room worked through with them.

Check 2

Adjust for sun, people and cooking

Shade and sun. Take off 10 percent for a heavily shaded room, and add 10 percent for a very sunny one.

People. If more than two people regularly use the room, add 600 BTU for each extra person.

A kitchen. Add 4,000 BTU.

A tall ceiling. The chart assumes 8 feet. ENERGY STAR says to go higher for a taller room.

A corner window. ENERGY STAR says that near a corner, look for a unit that can send its air in the right direction.

Getting the most from it

Check 3

Set it and place it well

The setting. ENERGY STAR says to set it as high as is comfortable, typically 78°F.

Humid days. Set the fan speed low. ENERGY STAR says slower air removes more moisture.

Heat near the thermostat. Keep lamps and TVs away from the unit’s thermostat, or it runs longer than it needs to.

Spreading the cool. ENERGY STAR suggests an extra fan to move the cooled air around the room: which way a ceiling fan should turn.

In winter. ENERGY STAR says to take the unit out at the end of the season if you can, or use a properly sized cover.

The refrigerant in a new one

Check 4Free to check

Look for a lower impact refrigerant

What ENERGY STAR lists. R-32, R-454B and R-290 as lower global warming refrigerants for room air conditioners.

Old stock. EPA’s table says new room units above its limit of 700, such as R-410A ones, could not be made or imported from 1 January 2025, and may be sold until 1 January 2028: the refrigerant phase out.

When it does not cool. A new unit of the right size that still struggles has a fault: a window AC not cooling.

Questions people ask

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

That is 144 square feet, so 5,000 BTU on ENERGY STAR’s chart, before adjusting for sun, shade or extra people.

How many square feet does a 10,000 BTU window unit cool?

ENERGY STAR gives 10,000 BTU for 400 up to 450 square feet with an 8 foot ceiling.

How many BTU for a 500 square foot room?

12,000 BTU, from ENERGY STAR’s 450 to 550 square foot band. Add 10 percent if the room is very sunny.

Is a bigger window air conditioner better?

No. ENERGY STAR says one that is too big cools before it removes the humidity and leaves the room damp and clammy.

How many BTU do I add for a kitchen?

4,000, ENERGY STAR says, and 600 for each person past two who regularly uses the room.

Does a portable AC follow the same chart?

ENERGY STAR’s chart is for room air conditioners. Portable units have their own quirks: a portable air conditioner not working.

Sources: ENERGY STAR "Room Air Conditioners" buying guidance, sizing chart and savings tips (specification effective 30 October 2023), and EPA's "Frequent Questions on the Phasedown of Hydrofluorocarbons"