Mini-Split Size Calculator
One row per room. Single-zone or multi-zone.
System
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Room estimates from the ENERGY STAR room AC chart; the insulation factor is our rough assumption. Whole-home systems need a Manual J load calculation by a contractor.
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How mini-split sizing works here
Each room's cooling load comes from the ENERGY STAR room AC chart (area bands, with −10% for heavy shade and +10% for strong sun). We then apply a rough insulation factor (+15% for poor insulation or a top floor, −10% for a well-insulated room). Indoor heads come in steps of 6,000, 9,000, 12,000, 18,000 and 24,000 BTU, so each room gets the smallest head that covers it.
Many store calculators silently round up to their smallest head, which can mean 30% or more oversizing for a small bedroom. We show the gap so you can decide. The outdoor unit should cover the total of the room loads; check the maker's table of allowed head combinations.
For whole-home systems, the DOE recommends a proper Manual J load calculation and Manual S equipment selection rather than rules of thumb. In cold climates, heating, not cooling, often sets the size: use the heat pump page to check capacity at your winter design temperature.
What size mini-split do I need for 500 square feet?
ENERGY STAR's room chart puts 500 sq ft at 12,000 BTU for average sun and insulation, so a 12,000 BTU (1-ton) head. Go up for a sunny, poorly insulated room or a kitchen, and check the heating capacity if you live somewhere cold.
Is it bad to oversize a mini-split?
Inverter mini-splits slow down to match the load, so a little oversizing is fine. A head much bigger than the room, beyond its lowest speed, short-cycles and removes less humidity. Check the unit's minimum capacity.
How do I size a multi-zone outdoor unit?
Add up the loads of the rooms, then pick an outdoor unit at least that big. Each maker publishes which combinations of indoor heads each outdoor unit accepts; check that your heads are on the list.
Sources: ENERGY STAR room AC chart; DOE Building America guidance on Manual J/S sizing. ACCA Manual J and S are copyrighted and not reproduced.