Mini-split sizing builder
Room size and climate → BTU → the cheapest units that fit. Enter each room you want to heat or cool. The builder estimates its cooling and heating load at your local design temperatures, checks each system's heating output when it's that cold outside, and prices one unit per room against a multi-zone kit.
How the sizing works
Cooling starts from the ENERGY STAR room-size table (about 20 BTU per sq ft for a typical room) and is adjusted for your summer design temperature (2% per °F away from 95°F), sun exposure (±10%), insulation, ceiling height, people beyond two (+600 BTU each) and kitchens (+4,000 BTU).
Heating is the room's heat loss at your winter design temperature, the temperature your area is colder than only 1% of the year: floor area × a loss coefficient for the insulation level × the gap between 70°F and that temperature.
Cold-weather output. A heat pump makes less heat as it gets colder. We use the maker's published capacity at 5°F where there is one and stop at its minimum operating temperature. Where the 5°F figure isn't published we assume 55% (75% for units on the NEEP cold-climate list) and label the result as an estimate.
Fitting. A unit fits when it covers at least 90% of the room's cooling load and no more than 1.8× it (oversized units short-cycle and leave rooms clammy). With "main heat" it must also cover the full heating load at the design temperature. Multi-zone heads are matched largest to largest and may run up to 2.5× their room's load.
Running cost uses typical full-load hours for your climate zone and each unit's SEER2 and HSPF2. Treat it as a ballpark: your thermostat habits matter as much as the hardware.
This is a rule-of-thumb estimate, not an ACCA Manual J. For whole-house primary heat in a cold climate, a Manual J (many utilities subsidise them) is worth the money before you spend thousands.