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Room air conditioner BTU calculator

Updated

Enter the room size and a few details. The calculator looks up the ENERGY STAR cooling capacity for that floor area and applies its adjustments for sun, people and kitchens.

Your room

How the calculator works

It follows the three steps on the ENERGY STAR room air conditioner page:

  1. Floor area = length x width, in square feet. Convert inches to feet first: 12 feet 6 inches is 12.5 feet.
  2. Chart capacity comes from the ENERGY STAR table below, based on an 8-foot ceiling.
  3. Adjustments: minus 10% if the room is heavily shaded, plus 10% if it is very sunny, plus 600 BTU/h for each person beyond two who regularly uses the room, and plus 4,000 BTU/h if the unit cools a kitchen.

The calculator applies the shade or sun percentage to the chart number, then adds the people and kitchen amounts.

Area to be cooled (sq ft)Capacity needed (BTU/h)
100 up to 1505,000
150 up to 2506,000
250 up to 3007,000
300 up to 3508,000
350 up to 4009,000
400 up to 45010,000
450 up to 55012,000
550 up to 70014,000
700 up to 1,00018,000
1,000 up to 1,20021,000
1,200 up to 1,40023,000
1,400 up to 1,50024,000
1,500 up to 2,00030,000
2,000 up to 2,50034,000

Source: ENERGY STAR.

Worked example

A 14 by 12 foot bedroom is 168 square feet, which falls in the 150 up to 250 row: 6,000 BTU/h. If it gets strong afternoon sun, add 10% for 6,600 BTU/h. With two people using it, there is no extra-person adjustment, so a unit rated around 6,000 to 7,000 BTU/h fits.

Why bigger is not better

ENERGY STAR warns that an oversized unit costs more, wastes energy and cools the room before it removes the humidity, so the air feels cool but damp. If your result falls between two unit sizes, pick the closer one rather than jumping up a size or two.

This chart is for single rooms. For a whole house, our central AC tonnage calculator gives a rough range, and a contractor's Manual J load calculation gives the real number.

Limits

Ceilings higher than 8 feet, large west-facing windows, poor insulation or an upstairs room under a hot roof can all need more capacity than the chart shows. The adjustments are ENERGY STAR's general guidance, not a load calculation.

Frequently asked questions

What size air conditioner do I need for a 12x12 room?
A 12 by 12 foot room is 144 square feet, which is in the 100 up to 150 row of the ENERGY STAR chart: 5,000 BTU/h. Add 10% if the room is very sunny, or take off 10% if it is heavily shaded.
How many BTUs do I need for a 300 square foot room?
The ENERGY STAR chart lists 300 up to 350 square feet at 8,000 BTU/h and 250 up to 300 at 7,000 BTU/h. A room of exactly 300 square feet falls in the 8,000 BTU/h row before any adjustments.
Does a kitchen need a bigger air conditioner?
Yes. ENERGY STAR says to add 4,000 BTU/h if the unit is used in a kitchen, because cooking adds heat.
What if my ceilings are higher than 8 feet?
The ENERGY STAR chart is based on an 8-foot ceiling, so a room with higher ceilings has more air to cool and may need a larger unit than the chart shows.

Sources

  1. Room Air Conditioners, ENERGY STAR