Mini-split sizing calculator (single & multi-zone)
Add each room you want to cool or heat. Each room gets its own indoor head, sized to the next standard capacity; the rooms together size the outdoor unit.
Size each zone at about 20 BTU per sq ft and round up to the next head size (6k, 9k, 12k, 18k, 24k). A single 12,000 BTU zone costs about $3,500–$5,000 installed; three zones about $6,500–$11,000.
| Zone | Load | Head size |
|---|
- Sourced installed range
- Interpolated (derived)
What size mini split do I need?
Size each room on its own, not the whole floor: every indoor head serves one zone. Use about 20 BTU per square foot, adjust for sun and kitchens, then round up to the next available head size.
- Measure each room’s floor area.
- Look up the base BTU on the ENERGY STAR chart.
- Adjust: −10% shaded, +10% very sunny, +4,000 BTU for a kitchen, and scale for ceilings above 8 ft.
- Round up to a standard head: 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 36,000 BTU.
- For multi-zone, the outdoor unit should cover the combined load. Manufacturers allow the connected heads to add up to more than the outdoor rating, since rooms rarely peak at once, but check the model’s allowed range.
Mini split installation cost by size and zones
| Single-zone size | Tons | Installed |
|---|---|---|
| 6,000 BTU | 0.50 ton | $2,500–$3,500 |
| 9,000 BTU | 0.75 ton | $3,000–$4,200 |
| 12,000 BTU | 1 ton | $3,500–$5,000 |
| 18,000 BTU | 1.50 ton | $4,500–$6,500 |
| 24,000 BTU | 2 ton | $6,000–$8,000 |
| 30,000 BTU | 2.50 ton | $7,000–$9,500 |
| 36,000 BTU | 3 ton | $8,000–$11,000 |
| 42,000 BTU | 3.50 ton | $10,000–$13,000 |
| 48,000 BTU | 4 ton | $12,000–$15,000 |
| Zones | Installed |
|---|---|
| 1 | $2,500–$6,000 |
| 2 | $4,500–$8,000 |
| 3 | $6,500–$11,000 |
| 5 | $10,000–$15,000 |
| 8 | $18,000–$20,000 |
Single-zone or multi-zone?
Single-zone (one outdoor + one indoor unit) is the simplest and most efficient choice for one problem room: a bonus room, garage office or addition.
Multi-zone runs 2–5 indoor heads from one outdoor unit, with separate temperature control per room. It saves wall space outside, but if the outdoor unit fails, every zone goes down with it. For far-apart rooms, two single-zone systems can be the better design.
Questions homeowners ask
What size mini-split do I need for 500 square feet?
A 12,000 BTU (1-ton) head covers about 450–550 sq ft. Go to 18,000 BTU if the room is very sunny or includes a kitchen.
How many square feet does a 9,000 BTU mini-split cover?
About 350–400 sq ft in average conditions.
Can I oversize a mini-split?
Inverter mini-splits handle mild oversizing better than single-speed units because they throttle down, but a head far too large for its room still short-cycles and dehumidifies poorly. Stay within one size step.
How much does a 3-zone mini-split cost?
About $6,500–$11,000 installed.
Estimates for planning only. Prices vary by region, home, and contractor. Gas, refrigerant, and high-voltage work must be done by a licensed technician.