Ductless Mini-Split Sizing Guide

Why Ductless Systems Need Different Sizing

Traditional central AC sizing uses a single whole-house BTU calculation. Ductless mini-splits flip this approach—each indoor head serves a specific zone with its own load requirements. This zone-by-zone precision delivers superior comfort and efficiency, but only if you size each head correctly.

The challenge: outdoor condensers come in fixed capacities (9k, 12k, 18k, 24k, 36k BTU), while indoor heads need to match individual room loads. Multi-head systems add complexity—you must balance total indoor capacity against outdoor capacity while avoiding over-serving or under-serving any zone.

Use our BTU calculator for each room first, then apply the strategies below to select equipment. Cross-check total loads with the multi-room planner to ensure your outdoor unit can handle the combined demand.

Step 1: Calculate Individual Zone Loads

Start with accurate BTU requirements for each room that will receive an indoor head:

Using the BTU Calculator

1. Measure each zone separately (living room, bedrooms, kitchen, etc.)
2. Input real data for each space using our BTU calculator:
   • Room dimensions (length × width × ceiling height)
   • Window area and orientation
   • Insulation quality
   • Sun exposure and shading
3. Note the recommended BTU range for each zone
4. Apply climate adjustments with the climate load adjuster

Example Zone Calculations

Step 2: Choose System Architecture

Single-Head Systems (1 Outdoor + 1 Indoor)

Best for: Single rooms, additions, garages, home offices

Sizing: Match indoor head capacity to room load within ±10%. Standard sizes: 9k, 12k, 15k, 18k, 24k BTU

Example: 400 sq ft bedroom needs 10,000 BTU → Choose 12k BTU system (closest match)

Multi-Head Systems (1 Outdoor + 2-8 Indoor Heads)

Best for: Whole-house or multi-room retrofits where ductwork doesn't exist

Sizing complexity: Must balance three factors:

1. Indoor total: Sum of all indoor head capacities
2. Outdoor capacity: Maximum BTU the condenser can deliver
3. Simultaneous load: Rarely do all zones need max cooling at once

Step 3: Select Outdoor Unit Size

For multi-head systems, outdoor unit capacity should cover 100-110% of your total calculated load, not necessarily 100% of your indoor head total:

Example Sizing Scenario

Calculated loads (from BTU calculator):

• Zone 1: 6,500 BTU
• Zone 2: 5,000 BTU
• Zone 3: 5,500 BTU
• Zone 4: 16,000 BTU
• Total actual load: 33,000 BTU

Indoor head selection (standard sizes):

• Zone 1: 9k head (slightly oversized, but smallest available)
• Zone 2: 6k head (perfect match)
• Zone 3: 6k head (perfect match)
• Zone 4: 18k head (matches load)
• Total indoor capacity: 39,000 BTU (118% of load)

Outdoor unit needed:

Choose 36,000 BTU outdoor unit. It covers 109% of actual load (33k BTU), even though indoor heads total 39k BTU. This works because:

• Zones rarely peak simultaneously
• Indoor heads can modulate below max capacity
• System prioritizes zones with active demand

Verify this approach with our multi-room planner, which calculates simultaneous load diversity factors.

Step 4: Distribution Strategies

Strategy 1: Exact Match (Ideal but Rare)

Indoor head capacities match zone loads precisely. Example: 12k load → 12k head. This rarely works perfectly due to limited head size options.

Strategy 2: Slight Oversizing (Common)

Indoor heads are 10-20% larger than zone loads. Most systems fall here because standard head sizes (6k, 9k, 12k, 15k, 18k) don't align perfectly with calculated loads. Acceptable if outdoor unit has sufficient total capacity.

Strategy 3: Load Diversity (Advanced)

Intentionally oversize indoor head total by 20-30% beyond outdoor capacity, banking on diversity. Example: 36k outdoor with 45k total indoor. Works for homes where bedrooms cool at night, living areas during day—never all zones at peak simultaneously.

Common Sizing Mistakes

Mistake 1: Oversizing Every Head

Error: Using 12k heads for 8k loads "to be safe"

Problem: Short cycling in individual zones, poor dehumidification

Fix: Use closest head size; 10-15% oversizing maximum per zone

Mistake 2: Undersizing Outdoor Unit

Error: 24k outdoor for 30k total load (80% coverage)

Problem: Can't satisfy demand on hot days; some zones under-cooled

Fix: Outdoor must be ≥100% of calculated load; verify with BTU calculator

Mistake 3: Ignoring Zone Diversity

Error: Sizing outdoor for 100% of indoor head total when loads don't overlap

Problem: Wasted capacity and higher upfront cost

Fix: Analyze usage patterns; use multi-room planner diversity factors

Mistake 4: Poor Head Placement

Error: Placing head at one end of L-shaped room or zone

Problem: Uneven cooling; "dead spots" away from head

Fix: Center heads in zones; consider two smaller heads for irregular spaces

Installation Questions for Contractors

Before signing a contract, ask these questions (reference our contractor bid comparison guide for full checklist):

1. What are the calculated BTU loads for each zone? (Should match your calculations)
2. Why did you choose this outdoor capacity vs. indoor head total? (Should explain diversity factors)
3. What's the indoor-to-outdoor capacity ratio? (110-125% typical; justify if higher)
4. How will the system prioritize zones under peak load? (Most systems serve all zones proportionally; some allow priority settings)
5. What's the line set run length? (Longer runs reduce capacity; confirm loads account for this)
6. Is each head individually controllable? (Should be yes; some cheap systems lack individual zone control)
7. What SEER2 rating does the outdoor unit have? (Higher = lower operating costs; see our SEER2 guide)

Ductless vs. Central AC: When to Choose Ductless

Compare costs and sizing using our central AC tonnage guide and this ductless guide to make an informed decision.

Next Steps

1. Calculate each zone: Use BTU calculator for every room
2. Sum total load: Add up all zones with multi-room planner
3. Select heads: Choose standard sizes closest to each zone load
4. Size outdoor: Ensure capacity ≥ total load (use 110-120% of load for diversity)
5. Get multiple bids: Compare proposals with bid comparison checklist
6. Verify efficiency: Check SEER2 ratings and operating costs