Room AC BTU Calculator
What size window or portable air conditioner do you need?
Cooling capacity
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Estimate based on the ENERGY STAR room air conditioner chart. Power assumes about 11 BTU per watt; check the unit's label.
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How the BTU estimate works
We use the ENERGY STAR room air conditioner chart, which gives a capacity for each band of floor area (for example 300 up to 350 sq ft → 8,000 BTU). Then we apply its four adjustments: heavily shaded −10%, very sunny +10%, +600 BTU for each person over two, and +4,000 BTU if the unit cools a kitchen. ENERGY STAR doesn't say which order to use; we apply the percentage first, then add people and kitchen.
Units come in steps (5,000, 6,000, 8,000, 10,000, 12,000…), so we suggest the smallest common size that covers the estimate and show how far above it that is.
| Area (sq ft) | BTU/h | Area (sq ft) | BTU/h |
|---|---|---|---|
| 100–150 | 5,000 | 550–700 | 14,000 |
| 150–250 | 6,000 | 700–1,000 | 18,000 |
| 250–300 | 7,000 | 1,000–1,200 | 21,000 |
| 300–350 | 8,000 | 1,200–1,400 | 23,000 |
| 350–400 | 9,000 | 1,400–1,500 | 24,000 |
| 400–450 | 10,000 | 1,500–2,000 | 30,000 |
| 450–550 | 12,000 | 2,000–2,500 | 34,000 |
Is 8,000 BTU enough for a 12×12 bedroom?
A 12×12 room is 144 sq ft, which ENERGY STAR puts at 5,000 BTU. An 8,000 BTU unit is well oversized for it unless the room is very sunny, has a kitchen or holds several people. A 5,000 or 6,000 BTU unit will dehumidify better.
Why is a bigger air conditioner not better?
An oversized unit cools the air quickly and switches off before it has pulled much moisture out, so the room ends up cold and clammy, and the unit cycles on and off more.
How many BTU per square foot?
ENERGY STAR's chart works out at roughly 20 BTU per square foot for typical rooms, a bit more for small rooms and less for large ones, before adjusting for sun, people and kitchens.
Portable AC ratings
Portable ACs now carry a DOE "SACC" rating, which is lower than the older ASHRAE BTU figure printed large on many boxes. Compare the SACC number with the estimate here.
Source: ENERGY STAR room air conditioners (US government).