What size mini-split do I need for each room?
Short answer: 9k for bedrooms, 12k for living rooms, 18k–24k for open plans — and the math below tells you when to break those rules.
The head sizes you can actually buy
Mini-split indoor heads come in fixed capacities: 9k, 12k, 15k, 18k, 24k, 30k, 36k BTU/hr (the "k" means thousands). You size up to the nearest head at or above your calculated load. Use the room BTU calculator for the load, this table for the match:
| Room | Sq ft (8 ft ceiling) | Load, BTU/hr | Head size |
|---|---|---|---|
| Bedroom (12×12) | 144 | ~4,500 | 9k |
| Large bedroom / home office | 200–300 | 6,500–9,500 | 9k |
| Living room | 350–500 | 10,500–15,500 | 12k |
| Big living + kitchen open | 550–700 | 15,500–21,500 | 18k |
| Great room / open plan | 750–950 | 22,000–28,000 | 24k |
| Small apartment, one zone | 1,000–1,300 | 29,000–36,000 | 30k–36k or 2-zone |
Assumes average sun exposure and insulation. Sunny west-facing rooms and pre-1980 houses shift you up a size — the calculator above accounts for both.
Multi-zone vs several single-zone units
For two or three rooms, compare before defaulting to multi-zone. A single-zone 12k condenser + head runs ~$1,500–2,500 DIY-friendly; a two-zone condenser with two heads typically runs $4,000–6,000 installed. Multi-zone wins when outdoor space is limited or rooms need simultaneous different setpoints. But note: multi-zone condensers divide their capacity across active heads, and if all zones call at once each head gets less than its nameplate — one reason pros size multi-zone systems to the sum of room loads with attention to diversity, not just head count.
The oversizing trap, quantified
Fixed-speed AC dehumidifies in the last half of each cycle, after the thermostat is nearly satisfied. Oversize the unit and it satisfies the thermostat first, every time — cold, clammy air, and a compressor that cycles on and off instead of cruising. Inverter mini-splits throttle down instead of shutting off, which is why they forgive 10–20% oversizing. Past ~30%, even inverters short-cycle at minimum output and you've paid for capacity you can't use.
DIY vs pro install
Pre-charged "no-vacuum" DIY units (Mr Cool-style) have made single-zone installs genuinely doable — hang the head, drill one 3-inch hole, connect the line set, 240 V or 120 V circuit by an electrician. Pro-installed brands (Mitsubishi, Fujitsu, Daikin) still outperform on efficiency and longevity, and multi-zone systems are pro territory: refrigerant handling, vacuum pull, and charge weighing are not DIY skills.
Matched to the sizing math: the 12k single-zone DIY class is the best-value entry, 18k–24k for open plans, and line-set kits + wall brackets round out the install.
Run your own numbers in the HVAC load calculator, then check airflow needs in the duct CFM guide.