This calculator estimates the cooling capacity, in BTU per hour, for a room window air conditioner in the United States. It follows the EPA ENERGY STAR room air conditioner method. It is not a whole-house or central air-conditioning calculation, and it does not use ACCA Manual J or any other copyrighted residential load-calculation method.
Results are a purchasing estimate only. They are not a substitute for a licensed HVAC contractor, manufacturer instructions, or a full load calculation.
How to measure the room
Measure the floor area of the space you want to cool, in feet. Convert inches to fractions of a foot (for example, 12 ft 6 in is 12.5 ft; 9 in is 0.75 ft, 6 in is 0.50 ft, and 3 in is 0.25 ft). That guidance comes from ENERGY STAR.
- For a rectangular room, multiply length by width.
- For a triangular area, multiply length by width and divide by two.
- For an irregular shape, split it into rectangles and triangles, then add the areas.
ENERGY STAR’s example: a rectangular room 14 ft 9 in long and 10 ft wide is 14.75 × 10 = 147.5 square feet.
If two rooms are used as one space with a wide opening, measure the combined area you actually intend to cool.
The ENERGY STAR method
The steps below follow Room Air Conditioners and How To Choose the Right Sized Window AC. Both pages were checked on November 15, 2026. We do not add extra multipliers of our own.
1. Look up a base capacity from the official chart. The chart is based on an 8-foot ceiling.
| Area to be cooled (square feet) | Capacity needed (BTUs 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 |
Source: U.S. EPA ENERGY STAR, Room Air Conditioners, checked November 15, 2026.
If the area is under 100 square feet, the official chart does not list a smaller size. The calculator reports the chart’s lowest listed capacity, 5,000 BTU/h, and notes that this is the floor of the chart, not a custom calculation for a smaller room.
If the area is over 2,500 square feet, the calculator does not extend the chart. It tells you the area is outside the published range and that you should have a contractor perform a load calculation.
2. Adjust for shade or sun, people, and a kitchen, using only the ENERGY STAR adjustments:
- If the room is heavily shaded, reduce capacity by 10%.
- If the room is very sunny, increase capacity by 10%.
- If more than two people regularly occupy the room, add 600 BTU for each additional person.
- If the unit is used in a kitchen, add 4,000 BTU.
3. Round down to the nearest thousand. ENERGY STAR’s window-AC guide says that after you have a total, you should round down to the nearest thousand — that is the size to look for when you shop.
The capacities above are calculated for an 8-foot ceiling. If your ceiling is higher, the estimate may be low. This calculator does not apply an extra ceiling-height factor, because ENERGY STAR does not publish one on the pages we used.
Poor insulation, west-facing windows, or a hot roof over a top-floor room can also make the real load higher than this estimate. The calculator does not invent adjustments for those conditions. If they apply, treat the result as a starting point and consider a professional load calculation.
A short example
ENERGY STAR’s own area example is 147.5 square feet, which falls in the “100 up to 150” row, so the base capacity is 5,000 BTU/h. If that room is a typically sunny or shaded living space with two people and is not a kitchen, the adjustments may leave that base number unchanged. If, instead, three people regularly use the room, add 600 BTU, then round down to the nearest thousand when you choose a unit. Always start from your own measurements and the official chart.
Why bigger is not always better
ENERGY STAR emphasizes that a larger unit is not automatically better. An oversized air conditioner can cool the room before it has had time to remove humidity. The room then feels damp and clammy, and the unit wastes energy. A unit sized to the area you are cooling is more likely to be both comfortable and efficient.
Room units and whole-house systems are different
This tool is only for a room window air conditioner (a unit that cools one room or a similarly bounded space). It is not for central air conditioning, mini-split whole-home design, or a whole-house load calculation. Those jobs use different methods, including ones we do not copy here, such as Manual J.
When to call a contractor
You should have a licensed HVAC contractor do a load calculation, rather than rely on this estimate, when:
- The area is larger than 2,500 square feet, or the space is more than a single room
- You are sizing a central or whole-house system
- Ceilings are well above 8 feet, insulation is poor, or the room has heavy west sun or a hot roof
- The result will be used for a fixed installation where a wrong size is expensive to undo
Disclaimer
Results are for learning and purchasing estimates only. They are not a substitute for a licensed HVAC contractor, manufacturer instructions, or a full load calculation. An oversized unit may not dehumidify well. This method is for U.S. room window air conditioners and is not directly applicable outside the United States or to whole-house systems. If you use these results, you do so at your own risk.