BTU per Head

Notes

• A mini-split serves one room, so the sizing error has nowhere to go. Central air averages a mistake across the house; a head does not. Too large in a 400 ft² kitchen means short cycles, no dehumidification, and a room that reads the right temperature and feels wrong.
• 12k is a clean fit — 0% over the calculated load, well inside the normal step.
• Inverter heads are forgiving, but not infinitely. Unlike single-stage equipment they throttle down instead of cycling, which is why mini-splits tolerate imperfect sizing better than a central system. What they cannot do is run below their minimum capacity, and an oversized head sits at that floor all season — which is where the humidity complaints come from.
• The rate here is 30 BTU/ft², and that is a starting point. A room with new windows and good insulation can want 15; a bonus room over an unconditioned garage, exposed on five sides, can want 30. Ceiling height matters too — the figures assume 8 ft, and a vaulted ceiling adds proportionally.
• Heating capacity is a separate number. A head rated 12,000 BTU for cooling does not deliver 12,000 BTU of heat at 5°F outdoors — output falls sharply as it gets colder, and the derating curve is specific to the model. In a cold climate, size the head on the heating load and check the manufacturer's capacity table at your design temperature.
• Where the head goes matters as much as its size. High on an interior wall with a clear path across the room; never above a door where the airflow hits a wall, and never where furniture blocks the return. A correctly sized head in the wrong place performs like an undersized one.