BTU calculator: the ENERGY STAR sizing chart
Room air conditioners are rated in BTU per hour of cooling. ENERGY STAR publishes this chart for rooms with 8 ft ceilings:
| Area to cool | Capacity needed | BTU per sq ft (approx.) |
|---|---|---|
| 100–150 sq ft | 5,000 BTU | 33–50 |
| 150–250 sq ft | 6,000 BTU | 24–40 |
| 250–300 sq ft | 7,000 BTU | 23–28 |
| 300–350 sq ft | 8,000 BTU | 23–27 |
| 350–400 sq ft | 9,000 BTU | 22–26 |
| 400–450 sq ft | 10,000 BTU | 22–25 |
| 450–550 sq ft | 12,000 BTU | 22–27 |
| 550–700 sq ft | 14,000 BTU | 20–25 |
| 700–1,000 sq ft | 18,000 BTU | 18–26 |
| 1,000–1,200 sq ft | 21,000 BTU | 18–21 |
| 1,200–1,400 sq ft | 23,000 BTU | 16–19 |
| 1,400–1,500 sq ft | 24,000 BTU | 16–17 |
| 1,500–2,000 sq ft | 30,000 BTU | 15–20 |
| 2,000–2,500 sq ft | 34,000 BTU | 14–17 |
AC BTU calculator: adjusting for your room
ENERGY STAR then adjusts the chart for four things, and the calculator applies all of them:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people regularly in the room: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU for cooking heat.
Because the chart assumes 8 ft ceilings, the calculator also scales the result up for taller rooms — a 10 ft ceiling adds 25% more air to cool. The result is rounded to the next common window-unit size: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000 or 24,000 BTU.
BTU per square foot — and why bigger isn't better
A useful shortcut is about 20 BTU per square foot, but notice how the chart's rate falls as rooms get bigger: small rooms need more per square foot because walls and windows make up more of their surface.
Our firm advice is to buy the size the chart gives, not one size up. An oversized unit cools the air quickly and shuts off before it has run long enough to pull moisture out, leaving the room cold and clammy — and short cycling wears the compressor. If you're between sizes in a humid climate, choose the smaller one and run it longer.
For the running cost of the unit you pick, use the air conditioner cost calculator. Sizing heat instead of cooling? The heating BTU calculator uses climate zones. And if the room overheats because of a hot attic above it, the blown-in insulation calculator may be the better fix.
