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.