ENERGY STAR sizes a room air conditioner by floor area: 6,000 BTU/h for 150 up to 250 sq ft (14 up to 23 m²), 12,000 BTU/h for 450 up to 550 sq ft (42 up to 51 m²). Then take off 10% for heavy shade, add 10% for a very sunny room, 600 BTU per person over two and 4,000 BTU for a kitchen.
How to measure the room
The ENERGY STAR chart works from floor area, so all you need is a tape measure. It tells you to "multiply the full length of the room by its width" (ENERGY STAR room air conditioners).
- Measure the long wall from one finished wall surface to the other, at floor level. Round up to the next half foot.
- Measure the short wall the same way, at a right angle to the first.
- Type both into the calculator, or multiply them yourself and switch the tool to floor area.
- If a wide opening with no door joins another space, measure that space too. The unit will be cooling both.
- Count the people who use the room most days, and judge the sun: heavily shaded, normal or very sunny.
ENERGY STAR says its capacities "are calculated based on an 8-foot ceiling". A taller ceiling holds more air, so treat the result as a minimum there.
The ENERGY STAR sizing chart
This is the chart the calculator applies, row for row. Each row runs "up to" the next one, so a room that lands exactly on an edge, like 150 sq ft, uses the higher row. The last column is the common size you can buy at or above each row, before any adjustments.
| Area to cool (sq ft) | Area (m²) | BTU per hour | Common size to buy |
|---|---|---|---|
| 100 up to 150 | 9.3 to 13.9 | 5,000 | 5,000 |
| 150 up to 250 | 13.9 to 23.2 | 6,000 | 6,000 |
| 250 up to 300 | 23.2 to 27.9 | 7,000 | 8,000 |
| 300 up to 350 | 27.9 to 32.5 | 8,000 | 8,000 |
| 350 up to 400 | 32.5 to 37.2 | 9,000 | 10,000 |
| 400 up to 450 | 37.2 to 41.8 | 10,000 | 10,000 |
| 450 up to 550 | 41.8 to 51.1 | 12,000 | 12,000 |
| 550 up to 700 | 51.1 to 65.0 | 14,000 | 14,000 |
| 700 up to 1,000 | 65.0 to 92.9 | 18,000 | 18,000 |
| 1,000 up to 1,200 | 92.9 to 111.5 | 21,000 | 24,000 |
| 1,200 up to 1,400 | 111.5 to 130.1 | 23,000 | 24,000 |
| 1,400 up to 1,500 | 130.1 to 139.4 | 24,000 | 24,000 |
| 1,500 up to 2,000 | 139.4 to 185.8 | 30,000 | over 24,000 |
| 2,000 up to 2,500 | 185.8 to 232.3 | 34,000 | over 24,000 |
Area bands and BTU values: ENERGY STAR room air conditioner sizing chart. Common sizes: ENERGY STAR certified room air conditioner list. Metric areas are converted.
The four adjustments
After the chart, ENERGY STAR changes the number for sun, people and cooking (ENERGY STAR):
- Heavily shaded: "reduce capacity by 10 percent."
- Very sunny: "increase capacity by 10 percent."
- More than two people: "add 600 BTUs for each additional person" who regularly uses the room.
- Kitchen: "increase capacity by 4,000 BTUs."
The calculator takes the 10% from the chart number, then adds people and kitchen, in the order ENERGY STAR lists them. Then it rounds up to the next size you can actually buy.
The math, with your numbers
These lines follow whatever you typed into the calculator above.
The last line is a cross-check. The Department of Energy says "typically, you need 20 BTU for each square foot of living space" (DOE Energy Saver, room air conditioners). That rule lands below the ENERGY STAR chart for small rooms, so the chart is the number to shop by.
Worked example: a sunny bedroom
A 16 × 14 ft bedroom (4.88 × 4.27 m) is 224 sq ft (20.8 m²). That falls in the 150 up to 250 sq ft row, so the chart says 6,000 BTU/h.
The room faces west and gets hard afternoon sun, so add 10%: +600. Three people use it, one more than two: +600. It is not a kitchen. That makes 7,200 BTU/h. No common unit is sold at 7,200, so the next size up is 8,000 BTU, which is 0.67 ton.
Tons, watts and the sizes you can buy
One ton of cooling is 12,000 BTU per hour (Oklahoma State University Extension). One kilowatt-hour is 3,412 Btu (US Energy Information Administration), so BTU per hour ÷ 3.412 gives watts. Those watts are cooling output, not the power the unit draws from the wall.
The calculator only rounds to sizes that are common. On the ENERGY STAR list of 570 certified room air conditioners, eight capacities each have at least 20 models, and together they cover 484 of the 570:
| Size (BTU/h) | Tons | Watts of cooling | Certified models |
|---|---|---|---|
| 5,000 | 0.42 | 1,465 | 21 |
| 6,000 | 0.5 | 1,758 | 29 |
| 8,000 | 0.67 | 2,345 | 99 |
| 10,000 | 0.83 | 2,931 | 108 |
| 12,000 | 1 | 3,517 | 129 |
| 14,000 | 1.17 | 4,103 | 43 |
| 18,000 | 1.5 | 5,275 | 27 |
| 24,000 | 2 | 7,034 | 28 |
Model counts: ENERGY STAR certified room air conditioners, grouped by cooling capacity, checked October 7, 2026. Only 6 listed models are larger than 24,000 BTU/h.
The other 86 models are spread over 42 in-between sizes, such as 7,800 and 15,000. When one of those sits closer to your number, the result mentions it.
Why a bigger AC is not better
Rounding up to the next common size is fine. Going well past it is not. ENERGY STAR puts it plainly: "An oversized air conditioner costs more to purchase, wastes energy and does not provide better cooling," and "if the unit is too large, it will cool the room before it has a chance to remove the humidity" (ENERGY STAR).
The Department of Energy agrees: an oversized unit "will cool the room too quickly without dehumidifying it properly," while "a smaller, properly sized unit running for a longer period operates more efficiently" (DOE Energy Saver). A cold, clammy room is the usual sign.
Mini split BTU: size each zone
A ductless mini split is sized room by room. Each indoor unit (the head on the wall) serves one zone, and "many models support up to four indoor air-handling units connected to one outdoor unit" (DOE Energy Saver, ductless mini-split heat pumps).
DOE stresses that "proper sizing and location of each indoor unit are crucial", and that oversized air handlers "can result in short cycling, wasting energy." Wall heads come in steps: Mitsubishi Electric's current MSZ-WX and MSZ-HX wall units run from 9,000 to 24,000 BTU/h (Mitsubishi Electric wall-mounted units).
Run this calculator once per room for a first number for each head. Then let the installer confirm it with a load calculation, because a mini split also heats, and the heating load can be the larger one.
Heating BTU: a starting point, not an answer
ENERGY STAR's chart is for cooling only. There is no matching government chart for heating a room, because heat loss depends on insulation, windows and climate.
For electric room heaters, the heater maker Stelpro gives one rule: "the recommended power is 1.25 W per cubic foot," and notes that insulation changes the answer (Stelpro support and FAQ). With an 8 ft ceiling that is 10 W per sq ft, or about 34 BTU/h per sq ft.
For your 224 sq ft room with an 8 ft ceiling: 1,792 cu ft × 1.25 W = 2,240 W, or about 7,643 BTU/h of heat.
For a furnace, boiler or heat pump, the real answer is a room-by-room load calculation to ACCA Manual J, described below.
Window vs portable AC
The ENERGY STAR chart and the sizes above are for room air conditioners: window and through-the-wall units. Portable units are rated differently.
Since DOE's 2016 test procedure, portable units carry a Seasonally Adjusted Cooling Capacity (SACC), "adjusted for the effect of infiltration air and duct heat transfer" and weighted 80% at 83°F and 20% at 95°F outdoors (DOE portable AC test procedure). The old ASHRAE number was higher. TCL, a maker, gives an example: a unit "previously rated at 14,000 BTU" (ASHRAE) "is now considered a 10,000 BTU" (SACC) unit (TCL support).
Shopping for a portable? Compare the box's SACC figure, not the ASHRAE one, with the size this calculator tells you to buy.
Rooms too big for a room AC
The chart stops at 2,500 sq ft (232 m²). Past that, the calculator will not give a size: a room AC is not the right tool. It also warns when the result passes 24,000 BTU/h, the largest common room unit.
For whole houses, open floor plans and central systems, ask for a load calculation. ACCA calls Manual J "the national ANSI-recognized standard for producing HVAC equipment sizing loads," and says such a calculation "is required by national building codes and most state and local jurisdictions" (ACCA Manual J).
Check your local code. A new central system, heat pump or furnace usually needs a permit, and your building department may ask to see the Manual J load calculation. Local rules differ.
Questions people ask
What size AC do I need for a 12 × 12 room?
5,000 BTU/h. A 12 × 12 ft room is 144 sq ft, which is in ENERGY STAR's 100 up to 150 sq ft row. If it is very sunny, add 10%: 5,500, which rounds up to a 6,000 BTU unit.
How many BTU do I need for 500 sq ft?
12,000 BTU/h, from the 450 up to 550 sq ft row. That is a 1 ton unit, before any sun, people or kitchen adjustments.
How many square feet does a 10,000 BTU AC cool?
About 350 up to 450 sq ft in normal conditions. The 400 up to 450 row asks for 10,000, and the 350 up to 400 row asks for 9,000, which rounds up to 10,000.
Is 12,000 BTU the same as 1 ton?
Yes. One ton of cooling is 12,000 BTU per hour, so an 18,000 BTU unit is 1.5 tons and a 24,000 BTU unit is 2 tons.
Should I round up or down?
Round up to the next common size, and stop there. ENERGY STAR warns that an oversized unit costs more, wastes energy and leaves the room humid.
Does a mini split use the same chart?
Use it as a first check for each zone. Each indoor head should be sized for its own room, and the installer should confirm it with a Manual J load calculation.
How many BTU to heat a room?
For electric heat, about 34 BTU/h per sq ft with an 8 ft ceiling, from Stelpro's 1.25 W per cubic foot rule. For a furnace or heat pump, get a Manual J.
Sources
- ENERGY STAR, Room air conditioners (sizing chart, adjustments, 8 ft ceiling, oversizing)
- ENERGY STAR, Certified room air conditioners data set (570 models, counts by capacity)
- US Department of Energy, Energy Saver: Room air conditioners; Ductless mini-split heat pumps; Test procedures for portable air conditioners (2016)
- US Energy Information Administration, British thermal units
- Oklahoma State University Extension, Understanding energy efficiency measures for heating and cooling systems (BAE-1409)
- ACCA, Manual J residential load calculation
- Mitsubishi Electric, Wall-mounted indoor units
- Stelpro, Support and FAQ (heater wattage rule)
- TCL, Portable air conditioner ASHRAE and SACC BTU ratings
Checked October 7, 2026. Metric values are converted from feet and square feet.