Ductless Mini-Split Sizing Calculator

Per-room and total-system cooling BTU from zone sq ft, climate, insulation, sun, and occupants — with single-zone vs multi-zone head count

Enter up to five zones. Get recommended indoor-unit BTU per room, total outdoor capacity, and whether you need one outdoor unit or two. Method: sizing guide. Field failures: troubleshooting. Central air decision: comparison. Lookup tables: BTU chart. Whole-home cooling: BTU tonnage calculator. Humidity / short-cycle: oversized vs undersized AC. Heating twin: furnace sizing. Hub: HVAC tools.

Starting point only — not ACCA Manual J, Manual S, or a contractor load calculation. This tool estimates cooling BTU from climate midpoints, insulation, ceiling height, sun exposure, and occupants, then rounds each zone up to a common indoor-unit size. It does not account for window area, orientation, infiltration, latent load, or manufacturer capacity tables at your design outdoor temperature. Oversizing short-cycles and leaves humidity; undersizing never reaches set point. Use this to frame a quote — then pay for a real load calc before you buy.

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How to Use This Mini-Split Sizing Calculator

Pick the climate zone that matches your region (same five bands as the furnace calculator). Set insulation and average ceiling height. Add each conditioned room as a zone with square footage, sun exposure, and occupants. Calculate. Read per-zone recommended head size, total system BTU, and single-zone vs multi-zone guidance.

CLIMATE midpoints BTU/sqft: 1→32, 2→27, 3→24, 4→21, 5→19
INSUL: poor×1.15, average×1.0, good×0.90 | heightFactor = ceilingFt / 8
SUN: shaded×0.90, average×1.0, heavy×1.10 | occupantExtra = max(0, occupants−2) × 400
rawBtu = sqft × climateBtu × insul × heightFactor × sun + occupantExtra
recommended = round up to nearest of [6000, 9000, 12000, 15000, 18000, 24000, 30000, 36000]

Worked example — Zone 4, 300 sq ft bedroom, 8 ft, average insulation, average sun, 2 occupants rawBtu = 300 × 21 × 1.0 × (8/8) × 1.0 + max(0, 2−2)×400 = 6,300 BTU.
Round up to the next catalog size: 9,000 BTU indoor unit.
One zone → single-zone outdoor unit sized for that head (verify manufacturer pairing tables).

Why the knobs move the number: guide. Wrong-size symptoms unique to ductless: troubleshooting. Static lookup tables: BTU chart.

What This Number Is (and Is Not)

Each zone result is a rounded indoor-head capacity for cooling. The total is the sum of those recommended sizes — a planning total for outdoor capacity, not a guaranteed outdoor model number. Multi-zone outdoor units are selected from manufacturer tables that limit how heads combine; this tool does not enforce those combination charts.

It is not Manual J (room loads with windows, infiltration, and design temperatures) and not Manual S (equipment selection at your outdoor design condition). Heat-pump heating capacity in cold weather is a different curve — size cooling here, then verify heating tables for your climate separately.

Round-up can oversize small rooms. A 6,300 BTU raw load becomes a 9k head. That jump is why Manual J still matters before you buy — and why humidity problems show up on oversized ductless the same way they do on oversized central AC. See oversized vs undersized AC for the general short-cycle / humidity pattern.

FAQ

Do I size one outdoor unit to the sum of the heads?

As a planning total, yes — then check the manufacturer’s multi-zone combination chart. Many outdoor units cannot run every head at full catalog capacity at once. If total recommended exceeds ~48,000 BTU, plan on two outdoor units or a larger commercial cabinet.

Why are climate factors the inverse of heating?

Hotter zones (1–2) need more cooling BTU per square foot; colder northern zones (4–5) need less cooling capacity for the same room. Heating uses the opposite intensity on the same five bands — see the furnace sizing calculator.