Mini Split Size Calculator
Add each room as a zone with its area, sun and ceiling height, pick your climate and insulation, and get the head size for every zone, the multi-zone total, the outdoor unit that covers it, and the power each head draws at your SEER2.
Zones
name · sq ft · sun · ceiling ft
| Zone | Load | Head size | Draw at SEER2 20 |
|---|---|---|---|
| Living room | 10,000 BTU/h | 12K | 600 W |
| Bedroom | 6,000 BTU/h | 6K | 300 W |
Heads total
18,000 BTU/h
1.5 tons across 2 zones
Outdoor unit
18K
multi-zone condensers are rated 18K-48K; larger loads need two
Standard sizes
6 / 9 / 12 / 18 / 24 / 30 / 36 / 48
K BTU/h
Standard head sizes and the room they fit
Average conditions, 8 ft ceilings, moderate climate. Watts are full-output draw at the SEER2 shown.
| Size | Tons | Room area | Watts @ SEER2 16 | Watts @ SEER2 22 | Typical circuit |
|---|---|---|---|---|---|
| 6K BTU/h | 0.50 | up to 250 sq ft | 375 | 273 | 15 A / 240 V |
| 9K BTU/h | 0.75 | 250–400 sq ft | 563 | 409 | 15 A / 240 V |
| 12K BTU/h | 1.00 | 400–550 sq ft | 750 | 545 | 15 A / 240 V |
| 18K BTU/h | 1.50 | 550–800 sq ft | 1,125 | 818 | 20 A / 240 V |
| 24K BTU/h | 2.00 | 800–1,000 sq ft | 1,500 | 1,091 | 20–25 A / 240 V |
| 30K BTU/h | 2.50 | 1,000–1,300 sq ft | 1,875 | 1,364 | 25–30 A / 240 V |
| 36K BTU/h | 3.00 | 1,300–1,500 sq ft | 2,250 | 1,636 | 30 A / 240 V |
| 48K BTU/h | 4.00 | 1,500–2,000 sq ft | 3,000 | 2,182 | 40–50 A / 240 V |
Circuit sizes are typical; the nameplate's Minimum Circuit Ampacity and Maximum Overcurrent Protection govern.
Sizing a multi-zone system
- Size each room as its own zone with the calculator, rounding up to a standard head.
- Add the heads. That is the maximum the outdoor unit should ever be asked for.
- Decide whether the zones peak together. Open-plan living and kitchen: yes. Bedrooms versus living room: usually not.
- Pick an outdoor unit between 80% and 100% of the sum accordingly, within the manufacturer's allowed connection ratio.
- Check the smallest head: many multi-zone condensers cannot modulate low enough for a 6K head in a small bedroom, which then short-cycles. A separate single-zone unit for small rooms is sometimes the better design.
Heating with a mini split
Most mini splits are heat pumps, and the heating load in a cold climate is usually larger than the cooling load for the same room: 45-60 BTU/h per square foot against 20-25. Size the head for the larger of the two, and check the unit's rated capacity at 5°F or −15°F rather than at 47°F -- a "12K" head may deliver 7K at −5°F. Cold-climate models (often branded Hyper-Heat or similar) hold 70-100% of rated output down to −13°F and are worth the premium north of climate zone 5.
Frequently Asked Questions
What size mini split for a 300, 500 or 1,000 sq ft space?
Roughly: up to 250 sq ft a 6K or 9K head; 250-400 sq ft 9K; 400-550 sq ft 12K; 550-800 sq ft 18K; 800-1,000 sq ft 24K; 1,000-1,500 sq ft 30K-36K. Those are average rooms in a moderate climate with 8 ft ceilings; a sunny great room with a vaulted ceiling in Phoenix can need a size up.
Can one mini split cool a whole house?
A single head cools the room it is in and whatever air can circulate to it; closed bedrooms down a hall will not get much. A multi-zone system with a head in each main space is how mini splits cover a whole house, and the calculator adds the zones up so you can size the outdoor unit.
Should I oversize a mini split?
Less than you would think. Inverter mini splits modulate down to about 25-30% of rated capacity, so they tolerate mild oversizing better than fixed-speed units, but a 24K head in a 12K room still short-cycles, dehumidifies poorly and costs more. Size to the load; if you are between sizes and the space is used for heating in a cold climate, go up; if cooling in a humid one, go down.
How much power does a mini split use?
Rated BTU/h divided by SEER2 gives watts at full output: a 12K head at SEER2 20 draws about 600 W, an 18K about 900 W. Because inverters modulate, average draw over a day is usually half that or less. A 12K unit is normally on a 15 A, 240 V circuit; 18K-24K on 20 A; check the nameplate MCA and MOP.
Multi-zone: should the outdoor unit equal the sum of the heads?
Manufacturers allow the heads to add up to 100-130% of the outdoor unit's rating, since not every room peaks at once. If every zone will run flat out together on the hottest day, match the sum; if bedrooms are used at night and living areas by day, an outdoor unit at 80-90% of the sum is normal.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.