Short answer: a 300 square foot room needs about 7,000 BTU of cooling as a starting point. That figure comes from the EnergyStar room air-conditioner sizing chart, which most manufacturers follow. But the honest answer is it depends — the same 300 sq ft room can need meaningfully more or less once you account for how it’s built and used. Here’s how to land on the right number.
The starting point: the EnergyStar chart
Rather than a flat rule, EnergyStar maps room area to a base cooling capacity. For a room in the 250–300 square foot range, that base is 7,000 BTU per hour. A 300 sq ft room is a 15 × 20 ft space, a large bedroom, or a small studio — and 7,000 BTU is where you begin before adjusting.
Window air conditioners are sold in standard sizes (5,000, 6,000, 8,000, 10,000 BTU and up), so once you settle on a number you’ll usually round to the nearest standard unit that covers it.
The adjustments that move the number
The chart assumes an average room. Real rooms aren’t average, so adjust:
- Sun exposure — a room that bakes in afternoon sun needs about 10% more; a heavily shaded room about 10% less.
- Insulation — a well-insulated room holds temperature and can take about 10% less; a poorly insulated one needs about 10% more.
- Ceiling height — the chart assumes an 8-foot ceiling. Add roughly 1,000 BTU for each foot above that.
- Occupants — add about 600 BTU per person who regularly uses the room beyond the first two.
- Kitchens — ovens and cooktops add heat, so add about 4,000 BTU if the room is a kitchen.
So a sunny, poorly insulated 300 sq ft kitchen with a high ceiling could need well over 10,000 BTU, while a shaded, well-insulated bedroom might be comfortable closer to 6,000.
Why bigger isn’t better
It’s tempting to buy the largest unit you can afford, but an oversized air conditioner is a common mistake. It cools the air quickly and shuts off before it has run long enough to pull moisture out, leaving the room cold and clammy. The frequent on-off cycling also wears out the compressor and uses more electricity. An undersized unit has the opposite problem — it runs non-stop and still can’t keep up on the hottest days. Matching capacity to the room is the comfortable, efficient middle.
What about a mini-split or central AC?
The required cooling BTU is the same whatever the equipment — it’s the heat the room gains that sets it, not the type of unit. So this 7,000-BTU-ish figure applies whether you’re sizing a window unit, a portable AC, a through-the-wall unit, or a ductless mini-split. Larger whole-home systems are sized by adding up every room.
Get your exact number
The 7,000 BTU baseline is a good anchor, but the adjustments are where accuracy lives. Enter your room’s size and details — sun, insulation, ceiling, occupants, and whether it’s a kitchen — and the calculator gives you a right-sized cooling figure plus the nearest standard unit to buy.