What base BTU per square foot does this use?

It starts from a climate-zone base load (about 20 BTU/h per square foot for hot zones down to roughly 30 for cold zones for combined heat-pump duty), then multiplies by adjustment factors for ceiling height above 8 feet, the fraction of wall that is glazing, and the insulation level. This is a quick-sizing approximation, not a substitute for a full room-by-room Manual J.

Why size each zone separately?

In a multi-zone mini-split each indoor head must independently meet its room's load. Summing rooms and buying one big head would leave smaller rooms over-conditioned and unable to dehumidify. The tool sizes each head to its room and then totals the outdoor unit.

Is the outdoor unit just the sum of the heads?

Not exactly. Indoor heads are usually rated higher in total than the outdoor unit because not all rooms peak at once; this is called diversity. The tool reports the summed indoor load as the outdoor target, which is conservative. Confirm the manufacturer's allowed connection ratio for the specific outdoor unit.

Should I round up to the next head size?

Pick the nearest standard head (9k, 12k, 18k, 24k BTU/h) at or just above the calculated zone load, but avoid large oversizing. An oversized ductless head short-cycles and dehumidifies poorly, defeating the main comfort advantage of inverter mini-splits.

Does this replace a Manual J?

No. This is a fast field estimate for quoting and feasibility. For a permitted installation, perform a full ACCA Manual J load calculation that accounts for orientation, infiltration, internal gains, and local design temperatures.

Mini-Split Ductless System Sizing Calculator

The mini-split sizing calculator estimates the indoor head capacity for each zone and the outdoor unit size for a ductless system. It uses a modified Manual J quick-sizing approach so contractors can quote and check feasibility fast, then confirm with a full load calculation before install.

How it works

Each zone’s load is a climate base load scaled by construction factors:

zone BTU/h = area x base(climate zone)
             x ceiling factor
             x window factor
             x insulation factor

outdoor target BTU/h = sum of all zone loads
outdoor tons         = outdoor target / 12,000

The climate base load reflects how hard the equipment must work in your zone. Ceiling height above 8 feet adds load, more glazing adds load, and better insulation reduces it. Each zone is sized independently to its own indoor head, then summed for the outdoor unit.

Example and tips

A single 400 sq ft room in a mixed climate (base 25 BTU/h per sq ft), with standard 8 ft ceilings, average windows, and good insulation, needs roughly 400 x 25 = 10,000 BTU/h, which rounds up to a 12k head. Add a 200 sq ft bedroom and you size a separate 9k head, giving an 18k-class outdoor unit for the two zones. Resist the urge to oversize heads: inverter mini-splits deliver their best efficiency and humidity control when matched closely to the actual load.