A ductless head conditions one space, so the question is a room load rather than a whole-house load.
The starting point
Roughly 20 BTU per hour per square foot for cooling in a typical room, which puts common head sizes at:
| Head size | Rough area |
|---|---|
| 6,000 BTU | up to 300 sq ft |
| 9,000 BTU | 300–450 sq ft |
| 12,000 BTU | 450–600 sq ft |
| 18,000 BTU | 600–900 sq ft |
| 24,000 BTU | 900–1,200 sq ft |
That is a starting figure for a room with ordinary exposure in a mixed climate, and it moves in both directions.
What increases it
A lot of west or south glass, which is the largest single factor in most rooms.
High or vaulted ceilings, since the load is volumetric.
A room over a garage or a crawl space, where the floor is an exterior surface.
A room under an uninsulated attic with only a ceiling between it and 140°F air.
A sunroom or an addition with more exterior wall than an interior room.
A kitchen, for the appliance load.
Occupancy, in a home office with equipment or a room used by several people.
What decreases it
A well-insulated modern envelope.
Shading, natural or built.
An interior room with few exterior surfaces.
A cool climate with a short cooling season.
Why oversizing is the mistake to avoid
This matters more with ductless equipment than most people expect.
A head that is too large satisfies the setpoint quickly and turns down to its minimum output or cycles. Either way it runs less, and dehumidification depends on run time at a cold coil.
The result is a room that reads the right temperature and feels damp — the single most common complaint after a mini-split installation, and it comes from a head one or two sizes too large.
Inverter equipment modulates, which helps, but the modulation range has a floor. A 12,000 BTU head with a minimum output of 3,600 BTU cannot deliver 1,500 BTU on a mild day; it will cycle.
The heating side
In a heat pump the same head has to satisfy a winter load too, and in a cold climate the heating requirement is larger.
Sizing to the heating load will oversize for cooling. The usual answers are cold-climate rated equipment with a wide modulation range, or accepting a modest shortfall on the coldest days, or supplementary heat in that room.
The manufacturer's extended capacity tables at low outdoor temperatures are what this decision needs, not the nameplate rating.
The one calculation worth doing
A room-by-room Manual J, which for a single room is a short exercise: wall and ceiling areas and assemblies, window area by orientation, floor condition, infiltration and internal gains.
Many ductless installers will do it. It typically comes out below the rule of thumb, and the difference is exactly the oversizing that produces the humidity complaint.
The practical check afterwards
On a hot afternoon, the head should run for long stretches at low output rather than reaching setpoint in ten minutes and stopping.
Indoor relative humidity in the room should settle around 45–50%. Above 55% while the temperature is being held is the signature of a head that is too large for the space.
Open-plan spaces
A single head serving a large open area is the one case where the simple area rule breaks down most often.
Air from a wall-mounted head throws a limited distance, and a kitchen-dining- living space of 900 square feet conditioned by one 24,000 BTU head will be cold at one end and warm at the other regardless of the capacity.
Two smaller heads at opposite ends usually work better than one large one, and they modulate lower, which improves the humidity behaviour as well.
Head types
Wall-mounted is the default and the cheapest. Ceiling cassettes distribute in four directions and suit a square room with a suspended or accessible ceiling. Floor-mounted units suit rooms with low wall space under windows and are better for heating-dominated use, since the warm air starts at floor level.
The type does not change the capacity calculation, and it changes considerably how evenly that capacity is delivered.