Mini-Split Sizing Calculator
Enter the zone's square footage, climate, and occupancy — get the recommended ductless mini-split head size.
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Single-zone vs multi-zone systems
A single-zone mini-split pairs one outdoor condenser with one indoor head — simplest setup, most efficient per head, and the easiest to size (run this calculator once). A multi-zone system pairs one larger outdoor condenser with two to eight indoor heads of potentially different sizes, each independently controlled — more flexible for a whole-house retrofit, but the outdoor unit has to be sized for the heads' combined capacity, which a contractor typically derates slightly since not every zone runs at full demand simultaneously. Size each head with this calculator first; total those numbers to guide the outdoor unit conversation with your installer.
How to use this calculator
Enter the square footage of the specific zone the mini-split head will serve (not the whole house — each indoor head is sized to its own zone), climate, and occupancy. Ductless systems are sized per-zone, so a multi-zone install means running this once per head.
Why mini-splits use different standard sizes than window units
Mini-split heads come in a narrower set of standard sizes than window units — commonly 9,000, 12,000, 15,000, 18,000, 24,000, 30,000, and 36,000 BTU. The underlying sizing math is the same rule-of-thumb approach as a room BTU calculation; what changes is which standard size the result rounds up to.
Worked example
A 400 sq ft zone, warm climate, 2 occupants: 400 × 25 BTU/sq ft = 10,000 BTU calculated, which rounds up to a standard 12,000 BTU head.
Frequently asked questions
Do I size each mini-split head separately in a multi-zone system?
Yes — each indoor head is sized to the specific room or open area it serves, not a share of the whole house. Run this calculator once per zone.
Can one outdoor unit support multiple different-sized heads?
Yes, multi-zone outdoor condensers are designed to support several indoor heads of different sizes simultaneously, as long as their combined capacity stays within the outdoor unit's rated range — an HVAC contractor can confirm the right outdoor unit pairing.
How is this different from the BTU calculator?
Same underlying method, different standard size table — the BTU calculator rounds to window/portable AC sizes; this one rounds to mini-split head sizes, which step in different increments.
Rule-of-thumb estimate, not a substitute for a full load calculation. Reference only — consult a licensed HVAC contractor before purchasing or installing equipment.