Mini-Split Sizing Calculator
Size ductless mini-split heads and the outdoor unit for a multi-zone system.
About Mini-Split Sizing Calculator
Multi-zone systems exploit the fact that zones rarely peak simultaneously, so the outdoor unit is sized below the sum of the heads. Pushing diversity too low leaves zones unable to reach setpoint on design days. Mini-splits modulate, so slight oversizing is far less damaging than with single-stage equipment, but heavily oversized heads still cycle poorly at part load.
Formula
Mini-Split Sizing
Outdoor capacity = sum of head capacities × diversity factor
- diversity factor
- Not every zone peaks at once, so the outdoor unit can be smaller than the sum
- SEER
- Cooling output in BTU per watt hour of electricity across a season
Worked example
With the default values below, Mini-Split Sizing Calculator returns Outdoor Unit Capacity of 10,498. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of zonesaccepts 1–8
- 3
- Total conditioned areaminimum 1 m2
- 95 m2
- Cooling intensityminimum 20 BTU/m2
- 130 BTU/m2
- Diversity factoraccepts 40–100 %
- 85 %
- Seasonal efficiency ratiominimum 5
- 20
Results
- Outdoor Unit Capacity
- 10,498 BTU/h
- Sum of Indoor Head Capacities
- 12,350 BTU/h
- Capacity per Zone
- 4,117 BTU/h
- Outdoor Unit Capacity
- 3.077 kW
- Estimated Electrical Input
- 0.525 kW
Step by step
- 1Values used
- 2Mini-Split Sizing
- 3Outdoor Unit Capacity= 10,498 BTU/h
- 4Sum of Indoor Head Capacities= 12,350 BTU/h
- 5Capacity per Zone= 4,117 BTU/h
- 6Outdoor Unit Capacity= 3.077 kW
- 7Estimated Electrical Input= 0.525 kW
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Mini-Split Sizing calculated?
Outdoor capacity = sum of head capacities × diversity factor Multi-zone systems exploit the fact that zones rarely peak simultaneously, so the outdoor unit is sized below the sum of the heads. Pushing diversity too low leaves zones unable to reach setpoint on design days.
Why does Mini-Split Sizing matter?
Mini-splits modulate, so slight oversizing is far less damaging than with single-stage equipment, but heavily oversized heads still cycle poorly at part load.
What values do I need to enter?
This calculator takes 5 inputs: Number of zones, Total conditioned area, Cooling intensity, Diversity factor, Seasonal efficiency ratio. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.