Mini Split Calculator
Ductless mini splits only work well when sized correctly. Oversized units short cycle and fail to dehumidify; undersized units run constantly and never catch up. This mini split calculator uses the ACCA Manual J simplified 20 BTU per square foot base, then adjusts for ceiling height, climate zone, insulation, sun exposure, and occupants to recommend a standard mini split size.
Mini Split BTU Sizing
Start with a base of 20 BTU per square foot for cooling. Then apply these adjustments:
- Ceiling height: scale by actual height / 8 ft.
- Climate: hot zones need 15% more, cold zones 10% less (cooling-focused; heat pumps handle heating separately).
- Insulation: poor adds 25%, good subtracts 15%.
- Sun exposure: sunny south/west rooms add 10%, shaded rooms subtract 10%.
- Occupants: add 600 BTU per person above 2.
- Kitchen: add 4,000 BTU if the zone includes a kitchen.
Common Mini Split Sizes
| BTU | Covers (sq ft, average) | Common Use |
|---|---|---|
| 6000 BTU mini split | 100-150 sq ft | Small bedroom, den, office |
| 9000 BTU mini split | 150-250 sq ft | Bedroom, home office, small living room |
| 12000 BTU mini split | 250-450 sq ft | Living room, master bedroom, studio |
| 18000 BTU mini split | 450-700 sq ft | Great room, finished basement, garage |
| 24000 BTU mini split | 700-1,100 sq ft | Open-concept main floor, small home |
| 30000 BTU mini split | 1,100-1,500 sq ft | Whole-floor heating/cooling |
| 36000 BTU mini split | 1,500-2,000 sq ft | Large open area or small whole-home |
1 ton = 12,000 BTU. A 24000 BTU mini split is a 2 ton system; a 36000 BTU mini split is 3 tons.
Mini Split Size for a Bedroom
A 200 sq ft standard bedroom with average insulation and 8-ft ceilings needs about 4,000 BTU. The smallest common mini split is 6,000 BTU, so a 6000 BTU mini split is the typical pick. For bedrooms larger than 250 sq ft, step up to a 9000 BTU mini split.
Mini Split Size for a Garage
Garages vary widely based on insulation. An uninsulated 500 sq ft two-car garage in a mixed climate needs roughly 18,000 BTU due to the poor insulation factor. A well-insulated 500 sq ft garage needs only 12,000 BTU.
Multi-Zone vs Single-Zone
If total load exceeds 36,000 BTU or you want independent control of multiple rooms, use a multi-zone mini split: one outdoor condenser serving 2-5 indoor heads. Size each head for its room's load, then size the outdoor unit for 70-80% of the sum (diversity factor - not every head runs at full capacity simultaneously).
Heat Pump Mini Splits in Cold Climates
Modern cold-climate mini splits (Mitsubishi Hyper-Heat, Fujitsu XLTH, LG Extreme) maintain 80-100% of rated capacity at 5°F and produce usable heat down to -13°F. Standard mini splits lose capacity rapidly below 32°F. In climate zones 5-7, specify a cold-climate model and derate effective heating capacity by 10-20% when sizing.