A room air conditioner's cooling capacity is rated in BTU per hour. Getting it right matters both ways. ENERGY STAR points out that an oversized unit cools the room but removes only some of the humidity, leaving it damp and clammy, while an undersized one runs constantly and still can't keep up. This BTU calculator uses ENERGY STAR's room air conditioner sizing chart, from 5,000 BTU/h for 100–150 ft² up to 34,000 BTU/h for 2,000–2,500 ft². It then applies ENERGY STAR's adjustments: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU/h for each regular occupant beyond two, and +4,000 BTU/h for a kitchen.
Measure the room first; the square footage calculator handles L-shaped and odd rooms. The chart is meant for window, wall and portable room units, not for sizing central air for a whole house.
How to use the room air conditioner btu calculator
- Measure the floor of the room to be cooled. For an irregular room, split it into rectangles and triangles and add their areas, as ENERGY STAR suggests.
- Choose the sun exposure: very sunny for big south- or west-facing windows, heavily shaded for rooms under trees or on a north side.
- Enter how many people regularly use the room.
- Pick "kitchen" if the unit will cool a kitchen, where cooking adds a lot of heat.
- Enter the ceiling height; the chart is based on 8 ft ceilings.
Formula
The chart value comes from the room area band in the table below. People are only added above two, and the kitchen allowance is a flat 4,000 BTU/h. The "unit to look for" is the next common size at or above the result. For conversions, 1 BTU/h = 0.29307 W, and 12,000 BTU/h is one "ton" of cooling.
Worked example
A 16 × 14 ft bedroom with a big west window, used by three people.
- Area: 224 ft², which falls in the 150–250 ft² band, so the chart gives 6,000 BTU/h.
- Very sunny: 6,000 × 1.10 = 6,600.
- One person above two: +600 = 7,200 BTU/h.
- Look for a 7,000–8,000 BTU/h unit, leaning toward 8,000 BTU/h if the room heats up in the afternoon.
A 525 ft² living room with normal sun and two people falls in the 450–550 band, so 12,000 BTU/h, or one ton.
ENERGY STAR room AC sizing chart
| Area to be cooled | Capacity needed |
|---|---|
| 100 up to 150 ft² | 5,000 BTU/h |
| 150 up to 250 ft² | 6,000 |
| 250 up to 300 ft² | 7,000 |
| 300 up to 350 ft² | 8,000 |
| 350 up to 400 ft² | 9,000 |
| 400 up to 450 ft² | 10,000 |
| 450 up to 550 ft² | 12,000 |
| 550 up to 700 ft² | 14,000 |
| 700 up to 1,000 ft² | 18,000 |
| 1,000 up to 1,200 ft² | 21,000 |
| 1,200 up to 1,400 ft² | 23,000 |
| 1,400 up to 1,500 ft² | 24,000 |
| 1,500 up to 2,000 ft² | 30,000 |
| 2,000 up to 2,500 ft² | 34,000 |
Source: ENERGY STAR room air conditioner buying guidance. Capacities are based on an 8-foot ceiling. In this calculator, an area exactly on a boundary (such as 150 ft²) uses the lower band, and rooms under 100 ft² use the smallest size.
When a rule of thumb is not enough
Area charts can't see insulation, window area and orientation, air leakage or your climate. A sun porch in Phoenix and a basement room in Seattle with the same floor area need very different capacities. For central air or a heat pump serving a whole house, contractors should run a room-by-room load calculation (ACCA Manual J) rather than use square feet per ton. Install the unit as the instructions describe, using the insulation panels supplied: ENERGY STAR notes that a poorly installed room AC can leak as much air as a 6-square-inch hole.
Caveats and assumptions
- The chart is ENERGY STAR guidance for room units; it is not a load calculation.
- Climate, insulation, windows and electronics in the room can change the real requirement.
- Tall ceilings add air volume; the chart assumes 8 ft, so consider the next size up for higher ceilings.
- Check the outlet and circuit: larger room units may need a dedicated or 230 V circuit.
Frequently asked questions
What size air conditioner do I need for a 12 × 12 room?
144 ft² falls in the 100–150 ft² band, so 5,000 BTU/h, with adjustments for sun, people or a kitchen.
How many BTU do I need per square foot?
There isn’t a single figure. The ENERGY STAR chart ranges from about 33–50 BTU/h per ft² in the smallest band down to about 14–17 in the largest, because small rooms need more per square foot. Use the chart band rather than a single number.
What size AC for 500 square feet?
500 ft² falls in the 450–550 band, so 12,000 BTU/h before adjustments.
Is a bigger air conditioner better?
No. ENERGY STAR notes that an oversized unit cools the room but removes only some of the humidity, leaving it damp and clammy.
How much should I add for extra people or a kitchen?
Add 600 BTU/h for each regular occupant beyond two, and 4,000 BTU/h if the unit cools a kitchen.
Sources
- ENERGY STAR — Room Air Conditioners: sizing chart and adjustments — chart, 8 ft ceiling basis, shade/sun/people/kitchen adjustments
- ENERGY STAR — Certified Room Air Conditioners Fact Sheet — oversizing and installation air-leak notes
- NIST SP 811, Appendix B — conversion factors — BTU/h to watts
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.