Calculator 01
Mini-split BTU sizing calculator
A quick, transparent estimate for one zone (one indoor unit). Not a Manual J load calculation.
| Kit | Rated | At 5°F | Install |
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"At 5°F" = manufacturer-published heating capacity at 5°F outdoor / 70°F indoor; "n/p" = not published on the product page we cite. Specs checked Oct 19, 2026.
How this calculator works
Cooling uses the ENERGY STAR room-sizing chart (which assumes 8-foot ceilings), scaled for your ceiling height, plus ENERGY STAR's own adjustments: −10% for heavy shade, +10% for a very sunny room, +600 BTU for each regular occupant beyond two, and +4,000 BTU for a kitchen.
Heating uses a rule-of-thumb heat loss per square foot by climate zone. These factors are our labeled assumptions, not a published standard: about 25 BTU/h per sq ft in zones 1-2, 30 in zone 3, 35 in zone 4, 40 in zone 5, 45 in zone 6, and 55 in zones 7-8, for an average house with 8-foot ceilings. We then multiply by 1.25 for poor insulation or 0.8 for a tight, well-insulated space, and scale for ceiling height.
The recommended size is the smallest common nominal size (9k, 12k, 18k, 24k, or 36k BTU) that covers the larger of the two needs. Then we check the heating need against what each kit's spec sheet says it delivers at 5°F, because a heat pump's output drops as it gets colder outside. See what mini-splits deliver at 5°F.
This is an estimate. For a whole house, a big addition, or a very cold climate, get an ACCA Manual J load calculation from an HVAC contractor or energy auditor. Oversizing wastes money and dehumidifies poorly in summer; undersizing leaves you cold on the worst nights.
Next step: estimate the yearly bill in the heat pump vs. gas, propane, and baseboard cost calculator.