Sizing a 2-zone ductless mini-split system means matching two indoor handlers (for example a living room and a master bedroom) with a dual-port outdoor condenser. This calculator applies ACCA cooling-load factors and a realistic 5% diversity factor, so you get the right outdoor unit without overspending on an oversized one.
🌀 Size Every Room & Zone of Your Ductless System
Sizing an entire multi-zone mini-split system? Add each room, get the recommended indoor handler size per room, and the matching outdoor condenser BTU & tons — using ACCA-style cooling-load rules and market-standard equipment sizes (6k / 9k / 12k / 18k / 24k / 30k / 36k BTU indoor).
1. System type
Single-zone means one indoor unit per outdoor unit. Multi-zone means one outdoor condenser serves 2–5 indoor handlers (often the most cost-effective way to cool an entire home without ductwork).
2. Add each room
For every room you'll cool, add a card below. Use the area slider (50–1,500 sq ft) and pick the sun / insulation profile. Each room's BTU load is rounded UP to the next standard indoor handler size (6k, 9k, 12k, 18k, 24k, 30k, or 36k BTU).
Up to 5 zones per outdoor condenser. Beyond that, add a second outdoor unit.
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Recommended Sizing & Diversity Breakdown
Here is how this configuration sizes out in the calculator above. Each room’s load is rounded up to the next standard indoor handler, then the outdoor condenser is chosen using a real-world diversity factor (rooms rarely all peak at once):
| Zone | Room Size | Recommended Indoor Handler |
|---|---|---|
| Living Room | 400 sq ft (Standard) | 9,000 BTU (0.75 Ton) |
| Master Bedroom | 350 sq ft (Standard) | 9,000 BTU (0.75 Ton) |
| Total Indoor Capacity | — | 18,000 BTU |
| Recommended Outdoor Condenser | Diversity factor 0.95 (5% reduction) | 18,000 BTU (1.5 Ton) |
How the Diversity Factor & Electrical Work
Why the outdoor unit is smaller than the indoor total. Add up the indoor handlers and you get 18,000 BTU, yet the recommended condenser is only 18,000 BTU. That is the diversity factor at work: in a real home, all zones almost never call for maximum cooling in the same minute, so manufacturers let the connected indoor capacity exceed the outdoor nominal capacity (a “combination ratio” of up to about 130%). Sizing the condenser to the diversified load (18,000 × 0.95 ≈ 17,100 BTU → 18,000 BTU) avoids paying for an oversized outdoor unit and prevents the short-cycling that hurts efficiency and comfort.
Electrical requirements. A 18,000 BTU (1.5 Ton) multi-zone condenser runs on a dedicated 208/230V circuit and typically calls for a 20-amp double-pole breaker and a nearby outdoor disconnect. Always confirm the minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP) printed on your specific unit’s nameplate, and have a licensed electrician verify your main panel has the spare capacity before installation.
Frequently Asked Questions
What size outdoor unit do I need for a 2-zone mini split? A 2-zone system typically requires an 18,000 BTU (1.5 Ton) or 24,000 BTU (2 Ton) dual-port outdoor condenser, depending on the combined square footage of both rooms.
What size breaker is needed for a 2-zone mini split? Most 18,000–24,000 BTU 2-zone outdoor condensers require a dedicated 208/230V circuit with a 20-amp or 25-amp double-pole breaker.
A heat pump is this cycle run in reverse — see the refrigeration cycle explained.
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Figures are reviewed by the Refcon HVAC engineering team using U.S. DOE, ENERGY STAR & ACCA Manual J data. Estimates only — confirm with a licensed pro.