Updated April 2026
What Size Mini Split Do I Need? BTU Sizing Guide With Costs
Quick Formula
Multiply your room square footage by 20 to 25 BTU. A 400 sq ft room needs a 9,000 to 10,000 BTU unit. Adjust upward for high ceilings, poor insulation, or heavy sun exposure. Use the calculator below for a precise recommendation with cost.
Room Size to BTU Chart
| Room Size | BTU Needed | Equipment Cost | Installed Cost |
|---|---|---|---|
| 150 - 300 sq ft | 9,000 | $700 - $1,200 | $2,000 - $3,000 |
| 300 - 500 sq ft | 12,000 | $900 - $1,600 | $2,500 - $4,000 |
| 500 - 700 sq ft | 18,000 | $1,200 - $2,200 | $3,500 - $5,500 |
| 700 - 1,000 sq ft | 24,000 | $1,600 - $2,800 | $4,500 - $7,000 |
| 1,000 - 1,400 sq ft | 36,000 | $2,200 - $3,800 | $5,500 - $9,000 |
Adjustment Factors
The base formula assumes 8-foot ceilings, average insulation, and moderate climate. Adjust for your specific conditions.
Ceiling height over 8 ft
+12.5% per extra foot
10 ft ceiling = +25% BTU
Heavy sun exposure
+10%
South or west-facing windows
Heavy shade
-10%
North-facing, tree cover
Poor insulation
+20 to 30%
Old home, single-pane windows
Kitchen area
+4,000 BTU
Cooking heat adds load
High occupancy
+600 BTU per person over 2
Body heat matters in small rooms
BTU Sizing Calculator
Why Correct Sizing Matters
Oversized Unit Problems
- Short cycling: Unit reaches temperature too fast, turns off, temperature rises, turns back on. Constant on/off cycling wastes energy and wears the compressor.
- Poor dehumidification: The unit does not run long enough to remove moisture from the air. The room feels cold and clammy.
- Higher cost: You paid more for a larger unit that performs worse than a properly sized smaller one.
Undersized Unit Problems
- Never reaches setpoint: The unit runs continuously but cannot cool or heat the room to the desired temperature on extreme days.
- High energy bills: Running at maximum capacity 24/7 uses more electricity than a properly sized unit running at partial load.
- Shortened lifespan: Constant full-load operation wears components faster.
Multi-Room Sizing
For multi-zone systems, size each room independently. The outdoor unit must handle the total BTU load of all connected indoor units.
Diversity factor: In practice, not every room runs at full capacity simultaneously. Most manufacturers allow the total indoor unit capacity to exceed the outdoor unit rating by 10 to 30%. A 36K BTU outdoor unit can typically support indoor units totaling 42K to 48K BTU. Your installer will calculate the appropriate diversity factor for your layout.
Brand comparison includes maximum zone counts and line set lengths by manufacturer, which affect multi-room system design.