To size a room air conditioner by hand or with the AC size calculator below, find the room's square footage on the ENERGY STAR sizing chart, then adjust. A 15 × 12 ft bedroom (180 sq ft) needs about 6,000 BTU.
Add 10 percent for a very sunny room or subtract 10 percent for a heavily shaded one, add 600 BTU for each person beyond two who regularly uses the room, and add 4,000 BTU for a kitchen.
Don't round up "just to be safe." An oversized unit cools the air before it pulls out the humidity, which leaves the room clammy. Enter your room above to get the right size.
How to measure the room
- Measure length and width in feet. Measure the space the unit has to cool. If an area opens onto the room without a door, like a dining nook off a living room, include it, since the air conditioner will be cooling it too.
- Split odd shapes into rectangles. Add them up with our square footage calculator. Then enter the total square footage as the length and 1 as the width.
- Count the people who regularly use the room. ENERGY STAR's adjustment applies to people who regularly occupy it, not occasional guests.
- Note the sun. Tick "Room gets a lot of sun" for a room with strong sun exposure, or "Room is heavily shaded" if trees or buildings block most direct sun. If you tick both, the calculator applies neither.
- Check the window and the outlet. ENERGY STAR notes that window models fit most double-hung windows, casement models for narrow vertical windows usually require removing a window panel, and built-in models go in a sleeve in the wall. Measure the opening before you shop.
The formula
The calculator uses:
- Area = room length × room width
- Base BTU = the ENERGY STAR chart value for that area
- Sunny room: base × 1.10. Heavily shaded room: base × 0.90
- People: + 600 BTU for each regular occupant beyond 2
- Kitchen: + 4,000 BTU
- The result is rounded to the nearest BTU
When the area falls exactly on a chart boundary, such as 150 sq ft, the calculator uses the smaller size. Rooms over 2,500 sq ft are beyond the chart, and the calculator suggests central air or multiple units.
Worked example (the calculator's default values)
A 15 × 12 ft room used by 2 people, with no sun, shade, or kitchen adjustment:
- Area: 15 × 12 = 180 sq ft
- Chart: 180 sq ft falls in 150 up to 250 sq ft, so the base is 6,000 BTU
- Adjustments: none (2 people is the chart's baseline)
- Result: 6,000 BTU
The calculator shows: "Look for an air conditioner rated about 6,000 BTU. Room: 180 sq ft (chart base 6,000 BTU)."
The same room with adjustments
If that room gets strong afternoon sun and 3 people use it regularly, the math is 6,000 × 1.10 = 6,600, plus 600 for the third person = 7,200 BTU. The calculator lists the adjustments: "Room: 180 sq ft (chart base 6,000 BTU), +10% for sun, +600 BTU for 1 extra person."
Room units come in set capacities, so pick the model closest to your result rather than jumping up a size or two. The adjustments already account for sun, shade, people, and cooking, and the next section explains why extra capacity works against you.
ENERGY STAR room air conditioner sizing chart
| Area to be cooled | Capacity needed | Example room |
|---|---|---|
| 100 up to 150 sq ft | 5,000 BTU | 10 × 12 ft (120 sq ft) |
| 150 up to 250 sq ft | 6,000 BTU | 12 × 15 ft (180 sq ft) |
| 250 up to 300 sq ft | 7,000 BTU | 15 × 18 ft (270 sq ft) |
| 300 up to 350 sq ft | 8,000 BTU | 16 × 20 ft (320 sq ft) |
| 350 up to 400 sq ft | 9,000 BTU | 18 × 20 ft (360 sq ft) |
| 400 up to 450 sq ft | 10,000 BTU | 20 × 21 ft (420 sq ft) |
| 450 up to 550 sq ft | 12,000 BTU | 20 × 25 ft (500 sq ft) |
| 550 up to 700 sq ft | 14,000 BTU | 25 × 25 ft (625 sq ft) |
| 700 up to 1,000 sq ft | 18,000 BTU | 30 × 30 ft (900 sq ft) |
| 1,000 up to 1,200 sq ft | 21,000 BTU | 30 × 36 ft (1,080 sq ft) |
| 1,200 up to 1,400 sq ft | 23,000 BTU | 35 × 38 ft (1,330 sq ft) |
| 1,400 up to 1,500 sq ft | 24,000 BTU | 30 × 48 ft (1,440 sq ft) |
| 1,500 up to 2,000 sq ft | 30,000 BTU | 40 × 45 ft (1,800 sq ft) |
| 2,000 up to 2,500 sq ft | 34,000 BTU | 45 × 50 ft (2,250 sq ft) |
Adjustments
| Condition | ENERGY STAR adjustment | On a 6,000 BTU base |
|---|---|---|
| Heavily shaded room | Reduce capacity by 10% | 5,400 BTU |
| Very sunny room | Increase capacity by 10% | 6,600 BTU |
| More than 2 regular occupants | Add 600 BTU per extra person | 6,600 BTU with 3 people |
| Unit used in a kitchen | Add 4,000 BTU | 10,000 BTU |
Window AC size by room dimensions
5,000 BTU fits a 10 × 10 or 12 × 12 ft room and 9,000 BTU a 20 × 20 ft room on the ENERGY STAR chart, whose capacities are calculated for an 8-foot ceiling. Here is what the calculator returns for common room sizes used by 2 people, with each ENERGY STAR adjustment applied on its own:
| Room (ft) | Area (sq ft) | Chart base (BTU) | Very sunny, +10% | Heavily shaded, -10% | Kitchen, +4,000 |
|---|---|---|---|---|---|
| 10 × 10 | 100 | 5,000 | 5,500 | 4,500 | 9,000 |
| 12 × 12 | 144 | 5,000 | 5,500 | 4,500 | 9,000 |
| 12 × 14 | 168 | 6,000 | 6,600 | 5,400 | 10,000 |
| 14 × 16 | 224 | 6,000 | 6,600 | 5,400 | 10,000 |
| 16 × 16 | 256 | 7,000 | 7,700 | 6,300 | 11,000 |
| 15 × 20 | 300 | 7,000 | 7,700 | 6,300 | 11,000 |
| 18 × 18 | 324 | 8,000 | 8,800 | 7,200 | 12,000 |
| 20 × 20 | 400 | 9,000 | 9,900 | 8,100 | 13,000 |
| 20 × 24 | 480 | 12,000 | 13,200 | 10,800 | 16,000 |
| 24 × 24 | 576 | 14,000 | 15,400 | 12,600 | 18,000 |
A 15 × 20 ft room (300 sq ft) and a 20 × 20 ft room (400 sq ft) sit exactly on chart boundaries, so they get the smaller size; if either is very sunny, its result (7,700 or 9,900 BTU) is closer to the chart's next size, 8,000 or 10,000 BTU. Add 600 BTU for each person beyond two who regularly uses the room. ENERGY STAR doesn't give a figure for taller ceilings; the note under the calculator says, "Ceilings over 8 ft, poor insulation, or hot climates may need more."
Why bigger isn't better
ENERGY STAR puts it plainly: if the unit is too large, it cools the room before it has a chance to remove the humidity, and cool, moist air feels damp and clammy. An oversized unit also costs more to buy, wastes energy, and doesn't cool any better. The Department of Energy's heating and cooling guide adds that oversized systems can be noisy and can lead to mold and poor indoor air quality.
An undersized unit has the opposite problem: it can struggle to keep up on the hottest days. That's why getting the adjustments right matters more than rounding up.
Room, portable, or central air?
- Window and through-the-wall units. This is what the ENERGY STAR chart is built for.
- Portable units. Their boxes can show two ratings. According to Honeywell, the Department of Energy introduced the SACC (seasonally adjusted cooling capacity) rating in October 2017. It's measured under warmer, more realistic conditions than the older ASHRAE number, so it comes out lower. Compare the SACC figure with your result.
- Central air and heat pumps. Whole-house systems need a load calculation. The Department of Energy recommends insisting that your contractor use ACCA Manual J to size the equipment. For a furnace, the furnace size calculator gives a rough estimate and turns a Manual J heat loss into the output and input to look for.
BTU and tons by square footage
Here is what the ENERGY STAR chart gives for common room and home sizes, before any adjustments. A ton of cooling is 12,000 BTU per hour, so tons = BTU ÷ 12,000. An area right on a chart boundary, like 1,500 sq ft, gets the smaller size, as in the calculator.
| Area | BTU per hour | Tons |
|---|---|---|
| 500 sq ft | 12,000 | 1 |
| 1,000 sq ft | 18,000 | 1.5 |
| 1,200 sq ft | 21,000 | 1.75 |
| 1,500 sq ft | 24,000 | 2 |
| 1,700 sq ft | 30,000 | 2.5 |
| 2,000 sq ft | 30,000 | 2.5 |
| 2,500 sq ft | 34,000 | 2.83 |
For a whole house, read this as a first estimate only. The chart was written for room units, and the Department of Energy notes that area rules of thumb ignore local climate and how well a house is sealed and insulated; it says one ton can condition 800 to 1,000 sq ft in a very efficient house. A Manual J load calculation by your contractor sets the real size.
AC tonnage chart: square feet per ton
600 sq ft per ton is the traditional estimate for homes, American Standard says, and Carrier's rule of thumb of 20 BTU of cooling per square foot comes to the same figure (12,000 ÷ 20 = 600). Central air conditioners come in half-ton steps from 1.5 tons (18,000 BTU) to 5 tons (60,000 BTU), Carrier says. The area each size covers under that estimate and under DOE's two figures, an old estimate of one ton per 400 to 500 sq ft and 800 to 1,000 sq ft per ton in a very efficient house:
| Size | BTU per hour | At 600 sq ft per ton | DOE old estimate, 400 to 500 sq ft per ton | DOE very efficient house, 800 to 1,000 sq ft per ton |
|---|---|---|---|---|
| 1.5 tons | 18,000 | 900 | 600 to 750 | 1,200 to 1,500 |
| 2 tons | 24,000 | 1,200 | 800 to 1,000 | 1,600 to 2,000 |
| 2.5 tons | 30,000 | 1,500 | 1,000 to 1,250 | 2,000 to 2,500 |
| 3 tons | 36,000 | 1,800 | 1,200 to 1,500 | 2,400 to 3,000 |
| 3.5 tons | 42,000 | 2,100 | 1,400 to 1,750 | 2,800 to 3,500 |
| 4 tons | 48,000 | 2,400 | 1,600 to 2,000 | 3,200 to 4,000 |
| 4.5 tons | 54,000 | 2,700 | 1,800 to 2,250 | 3,600 to 4,500 |
| 5 tons | 60,000 | 3,000 | 2,000 to 2,500 | 4,000 to 5,000 |
The same 3-ton unit fits anywhere from 1,200 to 3,000 sq ft depending on the figure, so treat any row as a first estimate: American Standard, Carrier, and DOE all point to a Manual J load calculation for the real size.
Mini split size by square footage
12,000 BTU covers about 450 to 550 sq ft with a mini split, Bryant says, the same range the ENERGY STAR room chart gives. Bryant's mini split chart, with tons and the area that Carrier's rule of thumb (room square footage × 25 BTU) gives for each size:
| Mini split size (BTU) | Tons | Room size, Bryant chart (sq ft) | At 25 BTU per sq ft, Carrier (sq ft) |
|---|---|---|---|
| 6,000 | 0.5 | 150 to 250 | 240 |
| 9,000 | 0.75 | 250 to 350 | 360 |
| 12,000 | 1 | 350 to 550 | 480 |
| 18,000 | 1.5 | 550 to 700 | 720 |
| 24,000 | 2 | 700 to 1,000 | 960 |
Size each indoor unit to its own room: Carrier says each indoor unit is sized independently to match that room's square footage, and that a multi-zone system can cool up to six rooms from a single outdoor unit. Both charts assume 8-foot ceilings (Carrier adds average insulation), and Carrier recommends a Manual J load calculation by a professional for the final size.
Installation and efficiency tips
- Check the voltage before you buy. ENERGY STAR notes that a standard household outlet is on a 115-volt circuit, large 115-volt units may need a dedicated circuit, and 230-volt units may need a special circuit. To run a window unit on a portable generator in an outage, the generator size calculator includes 5,000 and 10,000 BTU models.
- Seal the install. ENERGY STAR says an improperly installed room air conditioner leaks as much air as a 6 square inch hole. Use the insulation materials that come with the unit.
- Keep heat away from the thermostat. ENERGY STAR advises against placing lamps or TVs near the unit's thermostat, since the extra heat makes it run longer.
- Use the fan settings. ENERGY STAR suggests an extra fan to spread the cooled air around, and a low fan speed on humid days, because slower air movement removes more moisture.
- Compare efficiency. ENERGY STAR's criteria for room units are written in CEER (combined energy efficiency ratio), measured in BTU per watt-hour. Between two units of the right size, the one with the higher CEER uses less electricity for the same cooling. To estimate what a unit will use in kWh, enter its wattage in the electricity cost calculator.
Common mistakes
- Sizing for the whole floor. If the door stays closed, size for the room. If it opens onto other spaces, include them.
- Counting guests as occupants. Only add people who use the room regularly.
- Skipping the kitchen adjustment. A kitchen needs 4,000 BTU more than a bedroom of the same size.
- Buying a portable unit by its larger number. Use the SACC rating.
- Treating the chart as exact. It's a starting point based on floor area. The Department of Energy points out that area-based rules of thumb don't take into account local climate or how well a house is sealed and insulated, so a leaky, poorly insulated room or a very hot climate may need more than the chart shows. That's a reason to lean toward the upper size, not to jump several sizes. If the attic above the room is poorly insulated, the attic insulation calculator shows how much ENERGY STAR recommends adding.
Frequently asked questions
What size air conditioner do I need for a 12 × 12 room?
A 12 × 12 room is 144 sq ft, which falls in the ENERGY STAR range of 100 up to 150 sq ft, so about 5,000 BTU. Add 10 percent if it gets a lot of sun, or 600 BTU for each regular occupant beyond two.
How many square feet will a 10,000 BTU air conditioner cool?
On the ENERGY STAR chart, 10,000 BTU matches rooms of 400 up to 450 sq ft. In a very sunny room, the same unit covers less area, and in a heavily shaded one, a bit more.
How many square feet does a 12,000 BTU air conditioner cool?
450 up to 550 sq ft on the ENERGY STAR chart, before any adjustments, and Bryant gives the same 450 to 550 sq ft for a 12,000 BTU mini split. That's 1 ton of cooling (12,000 BTU per hour). A very sunny room of that size calls for 13,200 BTU after ENERGY STAR's 10 percent increase, and a kitchen that size for 16,000.
How many BTU do I need for 500 square feet?
About 12,000 BTU, the chart value for 450 up to 550 sq ft, before any adjustments for sun, shade, people, or a kitchen.
Is a bigger air conditioner better?
No. ENERGY STAR says an oversized unit cools the room before it removes the humidity, leaving it damp and clammy, and that it costs more, wastes energy, and doesn't cool any better.
Why does a kitchen need a bigger air conditioner?
Cooking adds heat. ENERGY STAR recommends adding 4,000 BTU if the unit will be used in a kitchen, so a 168 sq ft kitchen goes from 6,000 to 10,000 BTU.
Does ceiling height affect AC size?
8 feet is the ceiling height the ENERGY STAR chart is calculated for, so a room with a taller ceiling can need more than the chart shows. ENERGY STAR doesn't say how much more, and the calculator doesn't ask for ceiling height; its note says ceilings over 8 ft, poor insulation, or hot climates may need more. Bryant and Carrier say their mini split charts assume 8-foot ceilings too.
Does this work for central air?
No. It's for room units. For central air or a heat pump, the Department of Energy recommends insisting that your contractor use ACCA Manual J to calculate the right size.
What size AC unit do I need for my house?
A Manual J load calculation sets the size of a central air conditioner or heat pump; the Department of Energy says to insist that your contractor use it. As a first estimate, the ENERGY STAR room chart gives 24,000 BTU per hour, or 2 tons, for 1,400 up to 1,500 sq ft. DOE also cites an old rule of one ton per 400 to 500 sq ft (3 to 3.75 tons for 1,500 sq ft) that ignores climate and how well the house is sealed and insulated.
What size AC do I need for a 2,000 sq ft house?
3 to 3.5 tons by American Standard's guide, which says a 2,000 sq ft home would likely need a 3 or 3.5 ton unit depending on the climate; Carrier says a 3-ton unit (36,000 BTU) is typical. The ENERGY STAR room chart gives 2.5 tons (30,000 BTU per hour) for 1,500 up to 2,000 sq ft, and DOE's figures run from 2 to 2.5 tons in a very efficient house to 4 to 5 tons by the old rule of one ton per 400 to 500 sq ft. A Manual J load calculation sets the size.
How many square feet does a 3 ton AC unit cool?
1,800 sq ft by American Standard's chart, which works out to 600 sq ft per ton, the same as Carrier's 20 BTU per sq ft. DOE's old estimate of one ton per 400 to 500 sq ft gives 1,200 to 1,500 sq ft, and in a very efficient house, where DOE says one ton can condition 800 to 1,000 sq ft, it's 2,400 to 3,000. A 3-ton unit is 36,000 BTU per hour.
What size mini split do I need?
Size each indoor unit to the space it serves. For a single room, the ENERGY STAR chart is a reasonable starting estimate: a 15 × 12 ft bedroom (180 sq ft) comes to about 6,000 BTU, or half a ton. One outdoor unit can feed one indoor head or several. ENERGY STAR warns that a system too large or too small for its space will struggle to keep you comfortable, and says your contractor should verify the size with a Manual J calculation.
What size air conditioner do I need for 1,000 square feet?
About 18,000 BTU. On the ENERGY STAR chart, 18,000 BTU covers 700 up to 1,000 sq ft and 21,000 BTU covers 1,000 up to 1,200 sq ft; when an area lands exactly on a boundary, the calculator uses the smaller size. Adjust for sun, shade, people, and a kitchen.
What size portable air conditioner do I need?
Use the same chart and adjustments, then compare the result with the unit's SACC rating rather than its larger ASHRAE rating. A 15 × 12 ft room (180 sq ft) used by two people comes to about 6,000 BTU, so look for a portable rated about 6,000 BTU SACC. Honeywell explains that SACC reflects performance across a range of temperatures and humidity levels, which is why it reads lower than the ASHRAE number on the same unit.
What does SACC mean on a portable air conditioner?
According to Honeywell, the Department of Energy introduced the SACC (seasonally adjusted cooling capacity) rating in October 2017. It's measured under warmer, more realistic conditions than the older ASHRAE number, so it comes out lower. Compare the SACC figure with your result.
Does a window air conditioner need a dedicated circuit?
Sometimes. ENERGY STAR notes that a standard household outlet is on a 115-volt circuit, that large 115-volt units may need a dedicated circuit, and that 230-volt units may need a special circuit. Check the unit's requirements before you buy.
Sources
- ENERGY STAR: Room Air Conditioners (sizing chart and tips)
- ENERGY STAR: Room Air Conditioner Sizing and Selection Guide (PDF)
- U.S. Department of Energy: Guide to Home Heating and Cooling (PDF)
- Honeywell Air Comfort: What Do ASHRAE and SACC BTU Ratings Mean?
- ENERGY STAR: Room Air Conditioners Key Product Criteria
- Penn State EGEE 102: Air Conditioner Efficiency
- ENERGY STAR: Ductless Heating & Cooling
- American Standard: What Size AC Unit Do I Need?
- Carrier: What Size Air Conditioner Do I Need?
- Carrier: How Big of a Mini Split Do I Need?
- Bryant: What Size Mini Split Do I Need?
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