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Mini Split Sizing Calculator (Room-by-Room)

Size ductless mini-split indoor heads zone-by-zone and derive diversified outdoor condenser capacity using ASHRAE climate tiers, envelope multipliers, and standard 88% multi-zone load diversity.

Mini Split Zone Inputs

Multi-Zone Room Array

Zone / Room Rows

Zone 1: Living Room

Zone 2: Master Bedroom

Zone BTU/sqft factors follow simplified ACCA rule-of-thumb approximations. Outdoor condenser capacity applies an 88% multi-zone diversity factor for non-simultaneous peak loads. For precision equipment selection, a full Manual J load calculation is recommended.

Live Results

Total Connected Indoor Load

18,625.0 BTU/h

Recommended Condenser Capacity

16,390.0 BTU/h

(Diversified) — Calculated at an 88% non-simultaneous peak load variance factor.

Balanced Multi-Split Array — Decentralized zone loads and load diversity factors register perfectly inside modular inverter-driven multi-zone system metrics.

Zone-by-Zone Head Sizing

  • Living Room

    12,375.0 BTU/h

    Size: 18k BTU Head

  • Master Bedroom

    6,250.0 BTU/h

    Size: 9k BTU Head

How to Use This Mini Split Sizing Calculator

  1. Review default zone rows or add rooms. The calculator starts with two sample zones — Living Room and Master Bedroom. Use + Add Zone / Room Row to model additional spaces, or delete rows you do not need.
  2. Enter floor area and envelope parameters per zone. For each room, set floor area in square feet, ASHRAE climate tier (1–7), envelope insulation grade, and solar window density profile. These drive the per-zone BTU/h cooling load.
  3. Optionally enter proposed head sizes. If you have a specific indoor head BTU rating in mind, enter it to evaluate under-capacitated (red), inverter match stable (green), or oversized (yellow) zone profiles.
  4. Review diversified outdoor condenser capacity. The Live Results panel shows total connected indoor load, recommended diversified outdoor condenser BTU/h at 88% diversity, per-zone head tier matches, and a system-wide zoning performance badge.
  5. Evaluate system badge warnings. Red indicates total connected load exceeds 60k BTU on a single condenser. Yellow flags poor insulation on one or more zones. Green confirms a balanced multi-split array within standard modular limits.

Formulas & Example

Per-zone loads use simplified ACCA rule-of-thumb BTU/sq ft factors with envelope and solar window adjustments. Outdoor condenser sizing applies standard multi-zone diversity.

Per Zone:
  Room BTU/h = Area × Zone BTU/sq ft × Insulation × Windows
  Head Size  = next standard tier ≥ Room BTU/h (6k–36k)

System:
  Total Connected Indoor = Σ Room BTU/h
  Outdoor Condenser      = Total Connected × 0.88 (Diversified)

Proposed Head Override:
  RED    = Proposed < Room BTU/h
  GREEN  = Room BTU/h ≤ Proposed ≤ Room BTU/h × 1.20
  YELLOW = Proposed > Room BTU/h × 1.20

Worked Example

Default two-zone layout: Living Room (450 sq ft, Zone 4, average, many) and Master Bedroom (250 sq ft, Zone 4, average, average):

Living Room  = 450 × 25 × 1.00 × 1.10 = 12,375 BTU/h → 18k Head
Master Bed   = 250 × 25 × 1.00 × 1.00 =  6,250 BTU/h →  9k Head
Total Indoor = 18,625 BTU/h
Outdoor      = 18,625 × 0.88 = 16,390 BTU/h (Diversified)

Frequently Asked Questions

How is per-room mini-split BTU load calculated?▾
Each zone load equals floor area × zone BTU/sq ft (Zones 1–2: 20, 3–4: 25, 5–6: 30, 7: 35) × insulation multiplier (poor 1.20×, average 1.00×, good 0.90×, excellent 0.80×) × window density multiplier (few 0.95×, average 1.00×, many 1.10×). The result is matched to the next standard manufactured head size from 6k to 36k BTU.
What is the 88% diversity factor for outdoor condenser sizing?▾
Multi-zone ductless systems rarely peak simultaneously in every zone. An 88% non-simultaneous peak load variance factor is applied to the sum of all connected indoor BTU loads to derive recommended outdoor condenser capacity, reducing oversizing on the central inverter-driven outdoor unit.
What do the proposed head size RED, GREEN, and YELLOW statuses mean?▾
Red means the proposed head BTU is below the calculated zone load (under-capacitated). Green means the proposed head is within 0–20% above the calculated load (inverter match stable). Yellow means the proposed head exceeds 120% of calculated load, risking temperature overshoot and poor dehumidification.
When should I split into two outdoor condensers?▾
When total connected indoor load exceeds 60,000 BTU/h, the system badge turns red. Standard single-condenser multi-split manifolds are typically limited around this threshold. Split the architecture into two independent multi-zone condensers to maintain proper refrigerant distribution and inverter modulation headroom.
How many zones can this calculator model?▾
There is no hard row limit — add as many zone rows as needed. Each row calculates independently with its own climate tier, insulation grade, window profile, and optional proposed head override. Results update instantly as rows are added or removed.

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