Multi-zone or single-zone units?

Multi-zone uses one outdoor unit; singles are cheaper to repair and more efficient.

All rooms rarely need full output at the same time.

Smallest mini split head?

Usually 6,000 BTU/h (≈ 1.8–2 kW).

Multi-zone or separate units?

Multi-zone uses one outdoor unit; separate units are more efficient and independent.

Can a small outdoor unit run several heads?

Yes, if real simultaneous load is low and the maker allows it.

What if a head is oversized?

It short-cycles and dehumidifies poorly.

Mini split sizing calculator

Area of up to four rooms and a W per area ratio: you get the stock size of each indoor head and the multi-zone outdoor unit.

Total load

22178.921 BTU/h

Net measurement: 15764.094 BTU/h

Indoor unit, room 18530.354 BTU/h
Indoor unit, room 26824.283 BTU/h
Indoor unit, room 36824.283 BTU/h
Multi-zone outdoor unit (80% diversity)12624.924 BTU/h

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How it's calculated (visible formula)

kW per room = area × W/m² ÷ 1000 → next stock size; outdoor ≈ 80% of total load

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure each room to condition.
Pick a load ratio for insulation and sun.
Work out each room's load.
Give each room the next larger indoor head.
Size the outdoor unit for total load with diversity.

Worked example: Living room 215 sq ft, bedrooms 130 and 108 sq ft, 110 W/m²

2.2 kW → 9,000 BTU head (2.5 kW)
1.32 and 1.1 kW → 6,000–7,000 BTU heads
Load sum 4.62 kW × 80%

→ Heads of 9k + 7k + 7k BTU and an outdoor unit around 18,000 BTU.

Common mistakes

Adding head ratings to size the outdoor unit.
An oversized head in a small bedroom.
Not checking the maker's allowed combinations.

Pro tips

Check the head combinations each outdoor unit allows.
Keep line-set lengths within limits.
Place the outdoor unit with airflow and service access.
Favor quiet heads in bedrooms.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

Create quote Calculate margin & GST

FAQs

Multi-zone or single-zone units?: Multi-zone uses one outdoor unit; singles are cheaper to repair and more efficient.
Why 80%?: All rooms rarely need full output at the same time.
Smallest mini split head?: Usually 6,000 BTU/h (≈ 1.8–2 kW).
Multi-zone or separate units?: Multi-zone uses one outdoor unit; separate units are more efficient and independent.
Can a small outdoor unit run several heads?: Yes, if real simultaneous load is low and the maker allows it.
What if a head is oversized?: It short-cycles and dehumidifies poorly.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.