Multi-Zone Mini-Split Sizing Calculator
Size each indoor head of a multi-zone ductless system and see the total the outdoor unit has to cover, using the ENERGY STAR room sizing method for every zone.
Method reviewed October 25, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
- Total cooling need
- 22,000BTU/h
- Total cooling need
- 1.83tons
- Smallest common outdoor size covering the total
- 24,000BTU/h
3 zones need about 22,000 BTU/h (1.83 tons) in total; the smallest common outdoor size that covers it is 24,000 BTU/h.
- Indoor-unit sizes, the outdoor unit and which combinations are allowed all depend on the manufacturer. Check its combination tables and minimum capacities before choosing equipment.
- Total of suggested indoor units
- 24,000BTU/h
Cooling need by zone
- Zone 1 need
- 10,000BTU/h
- Zone 2 need
- 6,000BTU/h
- Zone 3 need
- 6,000BTU/h
Suggested indoor unit size
- Zone 1 indoor unit
- 12,000BTU/h
- Zone 2 indoor unit
- 6,000BTU/h
- Zone 3 indoor unit
- 6,000BTU/h
Show the working
- Zone 1 (ENERGY STAR room method)10,000 BTU/h → 12,000 BTU/h indoor unit
- Zone 2 (ENERGY STAR room method)6,000 BTU/h → 6,000 BTU/h indoor unit
- Zone 3 (ENERGY STAR room method)6,000 BTU/h → 6,000 BTU/h indoor unit
How the zones are sized
Each zone is sized like a single room in our AC BTU Calculator: the ENERGY STAR chart gives a capacity for the floor area, then it is adjusted for sun, extra people and kitchens, and scaled for ceilings above 8 feet. Each zone then gets the next common indoor-unit size at or above its need, and the zone needs are added up for the outdoor unit.
The calculator also shows the total connected capacity of the suggested heads. That figure is usually higher than the zone total, because heads come in steps.
Check the manufacturer’s combination tables
Multi-zone outdoor units only work with certain combinations of indoor units. Manufacturers publish tables listing the allowed heads, the minimum and maximum total connected capacity, and the capacity each head actually delivers in each combination. Many allow the connected capacity to exceed the outdoor unit’s rating, within limits they set.
Choose equipment from those tables. The sizes here are generic steps to plan with.
Watch the smallest zones
A small bedroom may need only 4,000–5,000 BTU/h, but the smallest indoor units are often 6,000 or 9,000 BTU/h, and an outdoor unit cannot always turn down far enough for one small zone. NEEP’s heat pump sizing guide warns that this leads to short cycling. Combining small rooms into one zone, or using a ducted head for several rooms, can help.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Ceiling height
- 8 ft
- Number of zones (indoor units)
- 3 zones
- Zone 1: floor area
- 400 ft²
- Zone 1: sun exposure
- Average
- Zone 1: people
- 2
- Zone 1: kitchen?
- No
- Zone 2: floor area
- 180 ft²
- Zone 2: sun exposure
- Average
- Zone 2: people
- 2
- Zone 2: kitchen?
- No
- Zone 3: floor area
- 180 ft²
- Zone 3: sun exposure
- Average
- Zone 3: people
- 2
- Zone 3: kitchen?
- No
Result
- Total cooling need
- 22,000 BTU/h
- Total cooling need
- 1.83 tons
- Smallest common outdoor size covering the total
- 24,000 BTU/h
- Total of suggested indoor units
- 24,000 BTU/h
3 zones need about 22,000 BTU/h (1.83 tons) in total; the smallest common outdoor size that covers it is 24,000 BTU/h.
Assumptions
- Each zone is sized with the ENERGY STAR room air-conditioner chart and its adjustments, exactly as in our AC BTU Calculator.
- Suggested indoor units are the next size up from the zone need, from a generic list (6k, 9k, 12k, 15k, 18k, 24k BTU/h). Outdoor units from 18k, 24k, 30k, 36k, 42k, 48k BTU/h.
Limitations
- Cooling-based. Heating capacity in cold weather can be the deciding factor; check each zone’s heating need and the outdoor unit’s low-temperature capacity (see the Heat Pump Balance Point Calculator).
- NEEP’s sizing guide warns against outdoor units whose minimum capacity is larger than the smallest indoor unit’s need. Too many small zones on one outdoor unit can short-cycle.
Frequently asked questions
How many zones can one outdoor unit run?
It depends on the model; common outdoor units serve two to five indoor units, and some up to eight. The manufacturer’s specifications list the number of ports and the allowed combinations.
Can the indoor heads add up to more than the outdoor unit?
Often yes, within limits that each manufacturer publishes. When all zones run at once, the outdoor unit shares its capacity between them, so each head delivers less than its rating.
Does this size the system for heating?
No. It sizes for cooling. In cold climates the heating load is often larger, and outdoor capacity falls as it gets colder. Check the heating side with the Heat Pump Balance Point Calculator.
Should every room get its own head?
Not necessarily. Very small or rarely used rooms can short-cycle a head that is too big for them. Open-plan areas, or a ducted unit serving several small rooms, are often a better fit.
Sources
- Room Air Conditioners — sizing chart — ENERGY STAR (U.S. EPA / DOE). Floor-area sizing chart (5,000–34,000 BTU/h) and the shade, sun, occupancy and kitchen adjustments.
- Guide to Sizing & Selecting Air-Source Heat Pumps in Cold Climates (rev. 04/08/20) — NEEP (hosted by U.S. DOE Building Science Education Solution Center). Size from the manufacturer’s extended performance (maximum capacity) data; ccASHP maximum capacities at 5 °F can corroborate it; avoid outdoor units whose minimum capacity exceeds the smallest indoor unit’s need.
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