How the calculator works
The size of a room air conditioner is its cooling capacity in BTUs (British thermal units) per hour. The calculator follows the sizing method ENERGY STAR publishes for room air conditioners. First it finds your floor area: length times width, in feet. Convert inches to parts of a foot, so 12 feet 6 inches is 12.5 feet. For an L-shaped room, split it into rectangles and add them up.
Next it looks up your area in the ENERGY STAR chart below to get a base capacity. Then it applies the four adjustments ENERGY STAR lists:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people regularly in the room: add 600 BTUs for each extra person.
- Unit used in a kitchen: add 4,000 BTUs.
ENERGY STAR does not say what order to apply these in. We apply the sun percentage to the chart value first, then add people and the kitchen, so the percentage is not applied to those fixed additions.
| Area to be cooled (sq ft) | Capacity needed (BTU per hour) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
Worked example: a sunny bedroom
Your bedroom is 15 feet by 14 feet, gets strong afternoon sun, and two people sleep in it.
- Area: 15 × 14 = 210 sq ft, which falls in the 150 up to 250 band: 6,000 BTU.
- Very sunny: 6,000 × 10% = +600, for 6,600 BTU.
- People: two, so no addition. Not a kitchen, so no addition.
- Recommended: 6,600 BTU per hour. The next size up on the chart is 7,000 BTU, so shop for about that.
To run it, a 7,000 BTU unit with a CEER of 11 draws roughly 7,000 ÷ 11 = 636 watts. Eight hours a day for 30 days is 153 kWh. At New York's July 2026 average residential price of 29.90 cents per kWh (EIA), that is about $46 a month. At the U.S. average of 18.31 cents, it is about $28.
A kitchen example shows how fast the adjustments add up. A 300 sq ft open kitchen and living room with normal sun and four regular occupants starts at 8,000 BTU, adds 1,200 for the two extra people and 4,000 for the kitchen, for 13,200 BTU. That points to a 14,000 BTU unit.
Why the right size matters
Bigger is not better. ENERGY STAR warns that an oversized unit cools the room before it has time to remove humidity, which leaves the air cool but damp and clammy, and it runs less efficiently. It also costs more to buy and wastes energy. Size to the chart, then adjust.
Edge cases
- High ceilings. The chart assumes 8-foot ceilings. ENERGY STAR says higher ceilings may call for a more powerful unit but gives no formula, so treat the result as a minimum.
- Under 100 sq ft. The chart starts at 100 sq ft. We show its smallest size, 5,000 BTU.
- Over 2,500 sq ft. The chart stops there. Large open spaces usually need more than one unit or a contractor's load calculation.
- Placement. ENERGY STAR notes that a unit mounted near a corner should be able to direct airflow toward the room.
- Efficiency rules. Federal minimum CEER standards for room air conditioners rose for units made from May 26, 2026 (10 CFR 430.32). For a common louvered window unit of 8,000 to 13,900 BTU without a heat pump, the minimum went from 10.9 to 16.0. The calculator's default CEER of 11 reflects an older unit; for a new one, enter the CEER from its EnergyGuide label.
What to do next
Not sure of your room size? Measure it with the square footage calculator. To compare an old AC with a new one, or see what a fan or dehumidifier costs, use the appliance energy cost calculator. Sharing an apartment? Split the higher summer bill with the utility bill split calculator, and see energy upgrades and rebates for NY homeowners before you buy.