BTU calculator: what size air conditioner do you need?
The 340 sq ft room this calculator opens on, 8 ft ceilings, average sun, two people, needs 8,000 BTU/h of cooling: a window unit rated 8,000 BTU, or a mini split head at 9,000, since heads only come in fixed sizes. Enter your own room and the same bracket method ENERGY STAR publishes runs again, with adjustments for ceilings, sun and occupancy.
Start from a room type
BTU chart by room size
Standard conditions (8 ft ceilings, average sun, two occupants, not a kitchen):
| Room area | Cooling load (BTU/h) | Window AC | Mini split |
|---|---|---|---|
| 100 – 150 sq ft | 5,000 | 5,000 | 6,000 |
| 150 – 250 sq ft | 6,000 | 6,000 | 6,000 |
| 250 – 300 sq ft | 7,000 | 8,000 | 9,000 |
| 300 – 350 sq ft | 8,000 | 8,000 | 9,000 |
| 350 – 400 sq ft | 9,000 | 10,000 | 9,000 |
| 400 – 450 sq ft | 10,000 | 10,000 | 12,000 |
| 450 – 550 sq ft | 12,000 | 12,000 | 12,000 |
| 550 – 700 sq ft | 14,000 | 14,000 | 18,000 |
| 700 – 1,000 sq ft | 18,000 | 18,000 | 18,000 |
| 1,000 – 1,200 sq ft | 21,000 | 24,000 | 24,000 |
| 1,200 – 1,400 sq ft | 23,000 | 24,000 | 24,000 |
| 1,400 – 1,500 sq ft | 24,000 | 24,000 | 24,000 |
| 1,500 – 2,000 sq ft | 30,000 | — | 30,000 |
| 2,000 – 2,500 sq ft | 34,000 | — | 36,000 |
Bedroom vs living room: why the same house needs two different units
A typical bedroom (about 150 sq ft, average sun, two people) needs roughly 5,000 BTU, a 5,000 BTU window unit or a 6,000 BTU mini split head. A living room in the same house (about 350 sq ft, sunnier, and busier with people) needs about 9,400 BTU, nearly double. Sun and occupancy do most of that work, not just the extra floor area.
| Room | Sq ft | Conditions | Load (BTU/h) | Window AC | Mini split |
|---|---|---|---|---|---|
| Bedroom | 150 | average sun, 2 people | 5,000 | 5,000 BTU | 6,000 BTU |
| Living room | 350 | sunny, 3 people | 9,400 | 10,000 BTU | 12,000 BTU |
Use the presets above the calculator to load either scenario, then adjust to your actual room.
How the calculation works
The starting point is the ENERGY STAR room air conditioner table, which maps floor area to cooling capacity in brackets (it's deliberately non-linear: small rooms need more BTU per square foot). Then the standard adjustments apply:
- Very sunny room: +10% · heavily shaded: −10%
- More than two people: +600 BTU per extra person
- Kitchen: +4,000 BTU (the range and fridge are heat sources)
- Ceilings over 8 ft: +5% per extra foot (our adjustment; the table assumes 8 ft)
The result is a cooling load. Buy the next unit size at or above it: window units come in 5k, 6k, 8k, 10k, 12k… steps and mini split heads in 6k, 9k, 12k, 18k, 24k, 30k, 36k BTU. If the next size up is more than about 30% over your load, prefer the smaller unit. Oversizing costs comfort and money.
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Frequently asked questions
How many BTU do I need per square foot?
For a typical room with 8-foot ceilings, cooling works out to roughly 20 BTU per square foot; but the relationship isn't linear. Small rooms need proportionally more (a 12×12 bedroom needs about 6,000 BTU, ~40 BTU/sq ft) because walls, windows and doors dominate small spaces. Large open areas need proportionally less. That's why this calculator uses the ENERGY STAR bracket table instead of a single per-square-foot multiplier.
What size AC do I need for a 12×12 room?
A 12×12 room is 144 sq ft, which lands in the 100–150 sq ft bracket: 5,000 BTU. If it's very sunny add 10% and step up to a 6,000 BTU unit. The smallest mini split head made is 6,000 BTU, so that's the mini split answer regardless.
Is a bigger air conditioner better?
No. An oversized unit cools the air fast but shuts off before it dehumidifies, leaving the room cold and clammy, and the constant starting and stopping (short cycling) wears the compressor and wastes electricity. Size to the load, not above it; see our short-cycling guide.
My room opens into a hallway or another room. What do I enter?
Cold air doesn't respect doorways. If the space you're cooling opens permanently into another area (an open-plan kitchen/living room, a room with a wide archway), add that area's square footage too. Only leave it out if a door will usually be closed.
Does this work for heating BTU too?
No. Heating loads depend heavily on your winter climate and run higher than cooling loads in most of the country. Use the heating BTU calculator for furnaces and cold-weather heat pump sizing.