What BTU per square foot means
BTU (British Thermal Units) measures how much heat an air conditioner must remove per hour. The common planning shortcut is 20–30 BTU per square foot of cooled floor area for residential spaces.
Example: a 200 sq ft room × 25 BTU = 5,000 BTU calculated load. Retail mini-splits start at 9,000 BTU — you round up to the next available head size, not down.
These numbers are planning estimates for DIY sizing — not a substitute for a licensed Manual J load calculation or permit work.
When to use 20, 25, or 30+ BTU per sq ft
Tiny homes with good spray-foam insulation often land closer to 20. RVs, sheds, and old cottages with drafty walls often need 30 or more.
- 20 BTU/sq ft: well-insulated, shaded rooms in mild climates
- 25 BTU/sq ft: average insulation, typical interior rooms — good default
- 30+ BTU/sq ft: sunny west-facing walls, poor insulation, kitchens, or hot-humid zones
- Add load for high ceilings (volume above 8 ft), extra occupants, and electronics
Put it together with a calculator
Multiply length × width for floor area, apply your BTU factor, then adjust for ceiling height, sun, and occupants. Our BTU calculator applies these factors so you do not have to memorize every exception.
Worked example: 14×16 living room
Floor area = 224 sq ft. At 25 BTU/sq ft baseline = 5,600 BTU. Add 10% for a west-facing window (sunny exposure) → 6,160 BTU. Add 15% for a 9 ft ceiling → ~7,100 BTU calculated load.
Retail mini-split heads jump 9,000 → 12,000 → 18,000 BTU. You would select 9,000 BTU here — not 18,000. See common sizing mistakes if you are tempted to oversize.
For tonnage comparisons, 9,000 BTU ≈ 0.75 tons. Use our tonnage calculator when shopping central AC specs alongside ductless heads.