AC unit size calculator
Sizing & LoadTonnage for a whole house, with the oversizing check most rules of thumb skip.
tons · sq ft
Window AC unit size calculator
This window AC unit size calculator runs the ENERGY STAR room air conditioner method exactly as published: measure the room, read the BTU/h off the chart, apply the sun, people and kitchen adjustments, then round down.
Room shape: Rectangle
Pre-filled with ENERGY STAR's own example room, 14 ft 9 in by 10 ft. L-shaped room? Split it into rectangles and add the areas.
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Shop for this size
Calculated 5,000 BTU/h, rounded down to the nearest thousand.
What size window AC do I need?
A room air conditioner is sold by its cooling capacity in BTU per hour. ENERGY STAR publishes a sizing chart for exactly this question: find the room's floor area, read the capacity beside it. The chart below is that table, unchanged. Its capacities assume an 8-foot ceiling.
| Area to be cooled (ft²) | Capacity needed (BTU/h) |
|---|---|
| 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 |
Sources: EPA ENERGY STAR, Room Air Conditioners — buying guidance (read 2026-10-18)
The four adjustments, then round down
- Heavily shaded room: reduce capacity by 10 percent.
- Very sunny room: increase capacity by 10 percent.
- More than two regular occupants: add 600 BTU for each additional person.
- Unit used in a kitchen: increase capacity by 4,000 BTU.
The percentages work on the chart figure; the people and kitchen lines are added on top. The last step is the easy one to miss: once you have a total, round down to the nearest thousand — that is the number to shop for. Rounding down, not up, is deliberate — it is the same logic that keeps a central system from being oversized, covered in how to size an AC unit.
Sources: EPA ENERGY STAR, Room Air Conditioners — buying guidance (read 2026-10-18) · EPA ENERGY STAR, How To Choose the Right Sized Window AC (2024-02-02) (read 2026-10-18)
How to measure the room
- Measure length and width in feet. Turn inches into fractions of a foot: 3 in = 0.25 ft, 6 in = 0.5 ft, 9 in = 0.75 ft.
- Square or rectangular room: length × width. Triangular room: length × width ÷ 2.
- Anything else: split it into rectangles and triangles, work out each, and add them.
- Measure the space the unit actually has to cool. If the room opens onto a hall with no door, the air will spill into it.
Worked examples
| Room | ft² | Chart | Adjusted | Shop for | Note |
|---|---|---|---|---|---|
| Bedroom, 14 ft 9 in × 10 ft | 147.5 | 5,000 | 5,000 | 5,000 | ENERGY STAR's own example |
| Same bedroom, west sun, 3 people | 147.5 | 5,000 | 6,100 | 6,000 | Sun and a third person lift it one size |
| Kitchen, 12 × 14 ft | 168.0 | 6,000 | 10,000 | 10,000 | The kitchen line does the work |
| Living room, 20 × 22 ft, shaded | 440.0 | 10,000 | 10,200 | 10,000 | Shade and people roughly cancel |
Why the bigger unit is the wrong unit
An oversized unit cools the room before it has a chance to remove the humidity, leaving the room damp and clammy. A window unit only takes moisture out while its coil is cold and running; a unit that satisfies the thermostat in a few minutes and shuts off never gets the chance. That is the reason the method rounds down, and why buying the next size up "for headroom" backfires.
Room unit or whole house?
The chart stops at 2,500 ft². Past that — or when you are cooling several rooms from one place — you are sizing a central system: use the AC unit size calculator for whole-house tonnage, or the BTU calculator for a room-by-room load. For a few rooms without ducts, a ductless head per room is the usual answer: the mini split sizing calculator sizes each zone, and ductless vs central air weighs the two.
Sources: EPA ENERGY STAR, How To Choose the Right Sized Window AC (2024-02-02) (read 2026-10-18)
Get the capacity you paid for
An improperly installed room air conditioner leaks as much air as a 6-square-inch hole — so use the side panels and insulation that come in the box, and set the unit level so the condensate drains. Keep lamps and televisions away from its thermostat. ENERGY STAR's suggested setting is 78 °F, or as high as is comfortable.
Some newer room units also use a lower-impact refrigerant: lower-GWP refrigerants used in room air conditioners include R-32 (HFC-32), R-454B and R-290. The R-454B transition guide explains why central systems are changing too.
Sources: EPA ENERGY STAR, Room Air Conditioners — buying guidance (read 2026-10-18)
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