Key takeaways
- This uses ENERGY STAR's table for one room, from 100 up to 2,500 square feet.
- Adjust 10% for sun or shade, add 600 BTU per person over two, and 4,000 BTU for a kitchen.
- It isn't for central air, which needs a load calculation.
Enter the room's area, occupants, sun and use. The result uses ENERGY STAR's table, which assumes an 8-foot ceiling.
How the calculator works
- Find the BTU figure for your room's area in the table.
- Multiply by 0.9 for a heavily shaded room, or 1.1 for a very sunny one.
- Add 600 BTU for each regular occupant beyond two.
- Add 4,000 BTU if the room is a kitchen.
ENERGY STAR doesn't state an order for these steps. We apply sun first, so tell us if you see a source that says otherwise. The order rarely changes which size you'd buy.
Worked example
A very sunny 320-square-foot room with three regular occupants:
- Table: 8,000 BTU
- Sun: 8,000 × 1.1 = 8,800
- One extra person: 8,800 + 600 = 9,400 BTU
Applying sun last would give 9,460 BTU. Either way, you'd look at units around 10,000 BTU.
ENERGY STAR's table
| Area to be cooled | BTU per hour |
|---|---|
| 100 up to 150 sq ft | 5,000 |
| 150 up to 250 sq ft | 6,000 |
| 250 up to 300 sq ft | 7,000 |
| 300 up to 350 sq ft | 8,000 |
| 350 up to 400 sq ft | 9,000 |
| 400 up to 450 sq ft | 10,000 |
| 450 up to 550 sq ft | 12,000 |
| 550 up to 700 sq ft | 14,000 |
| 700 up to 1,000 sq ft | 18,000 |
| 1,000 up to 1,200 sq ft | 21,000 |
| 1,200 up to 1,400 sq ft | 23,000 |
| 1,400 up to 1,500 sq ft | 24,000 |
| 1,500 up to 2,000 sq ft | 30,000 |
| 2,000 up to 2,500 sq ft | 34,000 |
A row includes its lower number, so exactly 300 square feet uses the 8,000 row. Sizes outside the table aren't covered, and we don't extend it.
Why not just use 20 BTU per square foot?
You'll see flat rules online, but they don't match the table. At 20 BTU per square foot, a 300-square-foot room gets 6,000 BTU, while ENERGY STAR's table gives 8,000.
Higher ceilings
The table assumes 8-foot ceilings, and ENERGY STAR says higher ceilings may call for more capacity without giving a number. Other sites publish adjustments, but we didn't find an authoritative one, so we don't apply one. If your ceiling is high, consider the next size up and ask the maker or retailer.
Before you buy
- Fit: measure the window opening.
- Power: check the unit's requirements against your outlet and circuit. An electrician can help with larger units, and we haven't researched circuit requirements in detail.
- Refrigerant: if a unit needs refrigerant work, that's for a technician. EPA says some R-22a products sold for DIY use are flammable and unapproved.
ENERGY STAR's administration is moving to the Department of Energy, with the transition due by July 2027, so this page's link may change. We read the table on November 2, 2026.
Frequently asked questions
What size AC do I need for a 500-square-foot room?
ENERGY STAR's table gives 12,000 BTU for 450 up to 550 square feet, before any adjustments.
Can I use this for central air?
No. See what size AC for my house for why.
Is bigger better?
Not automatically, which is why ENERGY STAR publishes a table. Choose the size your room calls for.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- ENERGY STAR: Room air conditioners
- EPA: Section 608 resources for homeowners
- EPA: ENERGY STAR