What size air conditioner do I need for a 300 sq ft room?
Check it with your own numbers
Open the BTU Calculator with these values
| Room length | 20 ft |
| Room width | 15 ft |
| Sun exposure | Average |
| Regular occupants | 2 |
| Is it a kitchen? | No |
| Recommended capacity | 6,000 BTU |
| Room area | 300 ft² |
| Tons (cooling) | 0.5 |
The rule behind it
The standard rule of thumb, used by the Energy Star room AC sizing chart, is about 20 BTU per square foot. A 20x15 room is 300 ft², so 300 × 20 = 6,000 BTU, or 0.5 ton.
When to size up
6,000 BTU is right for a bedroom or home office with average sun and one or two people. Bump it up if:
- West or south windows with no shade. Add 10%. Pick "Very sunny" in the calculator.
- More people. Add 600 BTU for each regular occupant past two.
- Top floor under a hot attic. Not in the formula, but in practice a top-floor room often needs the next size up.
- Vaulted ceiling. More air to cool. The rule assumes 8 ft ceilings.
Don't oversize
A 10,000 BTU unit in a 300 ft² room will cool it fast and shut off before it pulls the moisture out of the air. The room ends up cold and clammy. Short cycling also wears out the compressor. Stay close to the number.
What to buy
Window unit
6,000 BTU window units are common and run on a standard 115-volt outlet. Most draw about 5 amps, so they can share a bedroom circuit.
Portable
Portable units are rated differently. Look for the SACC (seasonally adjusted) rating, which is often a third lower than the big number on the box. A portable labeled 8,000 BTU (ASHRAE) might deliver about 5,000 BTU SACC. Buy by SACC.
Mini split
The smallest ductless heads are usually 6,000 or 9,000 BTU. A 6,000 BTU head is a good fit and also heats in winter.
Related questions
For other sizes, use the full BTU Calculator or browse all answers.