Multi-Zone Mini Split Sizing Calculator

Estimates the cooling load of up to five rooms, rounds each to a standard ductless head (9k, 12k, 18k or 24k BTU/h), and suggests a multi-zone outdoor unit.

For homeowners comparing quotes before asking an installer for a Manual J.

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Results will appear here once you enter your values.

How the estimate works

Each zone is sized on its own. The base BTU/h comes from the ENERGY STAR room air conditioner chart, where each band includes its upper bound: up to 150 sq ft is 5,000 BTU/h, over 250 up to 300 sq ft is 7,000, over 300 up to 350 is 8,000, over 400 up to 450 is 10,000, and over 1,400 up to 1,500 is 24,000. The result is multiplied by ceiling height divided by 8, then adjusted for sun, occupants and a kitchen.

Defaults are three zones of 300, 250 and 400 sq ft under 8 ft ceilings, mixed climate, average sun and two occupants each. Those rooms come to 7,000, 6,000 and 9,000 BTU/h, a total of 22,000. All three round to 9,000 BTU/h heads, so the connected total is 27,000. The smallest listed condenser that is at least 22,000 and holds 27,000 within 130% is the 24,000 BTU/h unit, a connected ratio of 112.5%. The mixed-climate heating screen is 22,000 BTU/h.

  • Head size: the nearest of 9,000, 12,000, 18,000 or 24,000 BTU/h, with ties rounded up.
  • Condenser: must cover the total cooling estimate and keep connected capacity within your percentage limit.
  • Limits: 1 to 5 zones, 100 to 2,500 sq ft each, ceilings 7 to 16 ft, 0 to 20 occupants, connection limit 100 to 150%.

What this tool is not

This is a planning estimate, not an equipment selection or a design. Real multi-zone systems have approved head combinations, minimum connected capacities and heating output that falls as outdoor temperature drops. The climate multiplier for heating is a screening assumption, so a cold-climate home should compare the manufacturer's capacity table at the local design temperature with a Manual J heating load.

  • Get a Manual J: ask a licensed HVAC contractor for an ACCA Manual J before purchase.
  • Check the match table: the allowed head mix and ratio depend on the exact outdoor model.
  • Rooms without doors: open floor plans may need to be treated as one zone.

Sources and next steps

Room chart and adjustments: ENERGY STAR, Room Air Conditioners sizing guidance (energystar.gov/products/room_air_conditioners). Connected-capacity example: Fujitsu General, multi-zone product catalog (130% indoor to outdoor limit for one product family). Design temperature background: ENERGY STAR Residential New Construction HVAC design temperature limits. Whole-house sizing standard: ACCA Manual J, 8th edition.

For a single window or portable unit, use the AC BTU calculator. For whole-house heat loss use the heating load calculator, and to estimate savings from a higher SEER rating use the SEER savings calculator.

Frequently Asked Questions

Common questions about the Multi-Zone Mini Split Sizing Calculator

It looks up the room area in the ENERGY STAR room air conditioner chart, which is written for an 8 ft ceiling. It then multiplies by ceiling height divided by 8, takes 10% off for a heavily shaded room or adds 10% for a very sunny one, adds 600 BTU/h for each occupant beyond two, and adds 4,000 BTU/h if the zone has a kitchen. The height step is this tool's own volume adjustment, not part of the ENERGY STAR chart.

Sources & References

International System of Units (SI): energy, work and heat

Energy, work and heat is measured in the joule (J); 1 cal = 4.184 J. Conversions between SI and other units use exact, internationally agreed factors maintained by NIST.
→Source: NIST — International System of Units (SI)

International System of Units (SI)

Authoritative definitions for energy, work and heat, from the BIPM SI Brochure (9th edition), the defining reference for the SI.
→Source: BIPM — SI Brochure (9th ed.)

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