How to size a mini split

Sizing a ductless mini split comes down to one question: how much heat does the room gain (or lose) at design conditions? Capacity is quoted in BTU/h — 9,000 BTU/h ≈ 3/4 ton, 12,000 BTU/h = 1 ton — and the goal is to match the room's load closely, because an inverter unit runs best in the middle of its modulation range, not at either end.

The screening method

A room-level screen starts from floor area and layers on the factors that move heat:

BTU/h ≈ area (sq ft) × base intensity × adjustments

• Base intensity: roughly 20–30 BTU/h per sq ft for cooling in much of the US; ~25 is a common middle for an average room with 8 ft ceilings.
• Ceiling height: loads scale with air volume; a 10 ft ceiling adds roughly a quarter over an 8 ft baseline.
• Sun exposure: strong afternoon sun or big west glazing can add 10–25%; a shaded north room subtracts.
• Envelope: tight, well-insulated construction subtracts; an uninsulated bonus room or sunroom adds substantially.
• Internal gains: kitchens add ~4,000 BTU/h; each regular occupant beyond two adds a few hundred.
• Heating-dominated climates: run the same screen for heating with the local design temperature difference — the larger of the two loads picks the unit, and cold-climate heat pumps lose capacity as outdoor temperature falls.

The mini split sizing calculator runs this screen — area, ceiling factor, design ΔT, sun, envelope quality and internal gains for both cooling and heating — applies a margin, and then snaps to the next standard size.

Standard mini split sizes

Ductless heads come in fixed nominal steps; you pick the next one above the screened load:

6,000 · 9,000 · 12,000 · 18,000 · 24,000 · 30,000 · 36,000 BTU/h

That is 1/2 ton through 3 tons. If the screen lands just over a step (say 12,800 on a 12,000), look at the inputs before jumping to 18,000 — a one-step oversize is a 50% jump at the small end of the range, which is exactly where short-cycling starts.

Worked example — 300 sq ft bedroom

A 300 sq ft bedroom with 8 ft ceilings, average insulation and moderate sun exposure, two occupants, no kitchen:

300 sq ft × 25 BTU/h·sq ft = 7,500 BTU/h

7,500 BTU/h sits above a 6,000 unit, so the selection is the next standard size: 9,000 BTU/h (3/4 ton), running at about 83% of nominal at design conditions — a healthy duty point for an inverter head. Make the same room a west-facing bonus room over a garage with 10 ft ceilings and weak insulation and the screen can reach 11–12,000 BTU/h, moving the pick to a 12,000 unit. The factors, not the floor area, decide the size.

Why oversizing backfires

• Short-cycling and idling. Inverter compressors modulate down to roughly 25–40% of rated output. A 24,000 BTU/h head on a 7,500 BTU/h room can never run lower than the load except by cycling off and on.
• Poor dehumidification. Moisture removal needs long runtimes with a cold coil. A unit that satisfies the thermostat in minutes leaves the air cool but clammy — the classic oversized-AC complaint.
• Efficiency and wear. Published SEER/HSPF assume steady modulating operation. Cycling burns that margin and cycles the compressor hardware.

Where the screen ends and Manual J begins

Per-square-foot screening is legitimate for shortlisting a single-zone head for one room. It is not a load calculation: it cannot see your window schedule, orientation, infiltration, duct gains or local 1%/99% design temperatures. For whole-house design, multi-zone systems, permits or anything marginal, have an ACCA Manual J done (or run one with proper software) and size from that. Use the mini split sizing calculator to sanity-check the result, then the BTU/GPM/ΔT calculator for hydronic cross-checks or the furnace BTU calculator when the heating side dominates.

Frequently asked questions

How many BTU do I need per square foot?

Common screening values run about 20–30 BTU/h per square foot for cooling, with ~25 as a frequent middle for an average room. Tight new construction can sit below 20; sunny rooms, kitchens, high ceilings and poor insulation push toward 30 or more. Treat any per-square-foot number as a starting screen, not a final answer.

What size mini split for a 300 sq ft room?

At ~25 BTU/h per square foot, a 300 sq ft bedroom screens at about 7,500 BTU/h, which lands on a 9,000 BTU/h (3/4-ton) unit — the next standard size up. A sun-baked 300 sq ft bonus room over a garage could justify 12,000 BTU/h, and a shaded, well-insulated room might be fine at 6,000.

Is it better to oversize or undersize a mini split?

Neither, but oversizing is the more common and more damaging mistake. An oversized unit short-cycles or idles at its minimum output, controls humidity poorly, and wears faster. Mini-split inverters modulate down only so far — typically to 25–40% of rated capacity — so a unit two sizes too big never runs in its efficient band.

Do I still need a Manual J calculation?

For whole-house systems, multi-zone layouts, new construction or anything permitted — yes. ACCA Manual J is the recognized room-by-room load method and accounts for orientation, glazing, infiltration and local design temperatures properly. Per-square-foot screens are for shortlisting equipment, not replacing the load calculation.