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AC BTU Calculator: What Size Air Conditioner Do I Need?

Enter your room size in feet, sun, people and kitchen use to see how many BTUs your window or wall air conditioner needs (ENERGY STAR chart).

Tool inputs stay in your browser · optional AI search sends your query

Good to know: Scope: the cooling size (BTUs per hour) for one room cooled by a room air conditioner, such as a window or through-the-wall unit, using the ENERGY STAR sizing chart (rooms from 100 to 2,500 square feet, capacities based on an 8-foot ceiling) and its four adjustments: heavily shaded room −10%, very sunny room +10%, 600 BTUs for each person over 2 who regularly uses the room, and 4,000 BTUs if the unit is used in a kitchen. Rooms under 100 sq ft use the smallest size in the chart (5,000 BTU/hr). Not included: heating and space heaters, central air, heat pumps and ductless mini-splits, portable air conditioners, whole-house load calculations, insulation, windows, climate zone, electrical outlets and voltage, prices and energy costs. Everything stays in this browser tab; nothing is sent or saved. Independent ToolsKo estimate, not an ENERGY STAR product. Source reviewed November 12, 2026.

How to use it (step by step)

  1. 1Measure the room length and width in feet (or feet and inches, like 12' 6"), or choose “I know the square feet” and type the area.
  2. 2Pick how sunny the room is, type how many people usually use it, and choose “Yes” if the unit will cool a kitchen.
  3. 3Read the BTUs you need, then shop for a unit whose BTU label is closest to that number. Bigger is not better: an oversized unit wastes energy and leaves the room humid.

Words to know

BTU per hour (BTU/hr)
How much heat an air conditioner can remove from a room in one hour. The number on the box, like 8,000 BTU, is its cooling size.
Cooling capacity
Another name for the BTU size of the unit: how strong it is at cooling.
Oversized unit
An air conditioner too big for the room. It cools fast and shuts off before it dries the air, so the room feels damp and it uses more power.
Square feet (sq ft)
The floor area of the room: length × width, both in feet. A 12 ft × 15 ft room is 180 sq ft.

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Sources & methods

References checked November 11, 2026. Read each tool’s scope and assumptions before relying on its result.

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