What size window AC do I need?

Where the numbers come from

The size comes from ENERGY STAR's room air conditioner sizing chart. Find the row for the room's square footage, then adjust:

• Heavily shaded room: reduce capacity by 10 percent.
• Very sunny room: increase capacity by 10 percent.
• More than two people regularly in the room: add 600 BTU for each extra person.
• A kitchen: add 4,000 BTU.

The chart's capacities are calculated for an 8-foot ceiling. ENERGY STAR also warns that bigger is not better: an oversized air conditioner costs more to buy, wastes energy and doesn't cool any better. If it's too large, it cools the room before it can remove the humidity, and the room feels damp and clammy.

Source: ENERGY STAR, Room Air Conditioners. Retrieved October 30, 2026.

The size to buy

Window units come in set sizes, so the tool picks the smallest one that covers your number. The sizes come from ENERGY STAR's certified list: every capacity sold as at least 5 certified models from at least 2 brands. The orange ones need a 230-volt outlet: every certified model in those sizes does.

Checked against the list on October 30, 2026.

Source: ENERGY STAR Certified Room Air Conditioners (U.S. EPA data).

Install it right

• ENERGY STAR says a badly installed room air conditioner leaks as much air as a 6-square-inch hole. Use the insulation pieces that come with the unit, and set it level so it drains.
• Plug it straight into a wall outlet. The U.S. Fire Administration: “Never use extension cords with appliances. Plug them directly into wall outlets.” (source)

Questions

How many BTU per square foot does a window AC need?

There's no single number. The chart steps up by room size: 5,000 BTU for 100 up to 150 sq ft, 8,000 BTU for 300 up to 350 sq ft, and 12,000 BTU for 450 up to 550 sq ft. Then sun, people and kitchens adjust it.

Is a bigger window AC better?

No. Per ENERGY STAR, an oversized air conditioner costs more to buy, wastes energy and doesn't cool any better. It cools the room before it removes the humidity, so the room feels damp and clammy.

Will it plug into a regular outlet?

Up to 15,000 BTU, most certified models run on 115 volts. On ENERGY STAR's certified list (October 30, 2026), every 18,000 and 24,000 BTU model needs 230 volts. Each card shows the plug its maker lists. Plug it straight into the wall: the U.S. Fire Administration says never use extension cords with appliances.

Do high ceilings change the size?

The chart is built on 8-foot ceilings. With higher ceilings, treat the answer as a minimum and have an HVAC pro run a load calculation (ACCA Manual J) before you buy.

Does this work for a portable air conditioner?

No. ENERGY STAR's sizing chart is written for room air conditioners, and its certified list holds window and through-the-wall units only. Portable units aren't on it, so this tool doesn't size or list them.

ENERGY STAR room air conditioner sizing chart

Area to be cooled	Capacity needed