What Size Mini Split Do I Need? (2026 Guide)

The 20 BTU rule of thumb

For rooms with 8-ft ceilings and ordinary insulation, plan on about 20 BTU per square foot of floor area: multiply the area by 20 for the baseline load, then round up to a standard unit size.

Adjust for sun, climate and people

The baseline assumes up to two people, shade and a mild climate. Tune it with these factors:

• Sun exposure: Moderate — multiply by 1.1; High (large south- or west-facing windows) — multiply by 1.25.
• Hot climate: if afternoons regularly top about 95°F, multiply by 1.25.
• People: add 500 BTU per extra person beyond two.

Adjusted BTU = area × 20 × sun factor × climate factor + 500 × (people − 2). Factors stack — west sun and a hot climate both apply.

Mini split sizes by room area

Hot climate? The chart applies the ×1.25 factor to the 20 BTU/sq ft baseline and rounds up to the next standard size:

Treat these as planning estimates — the real load also depends on windows, ceiling height, insulation and airtightness. Confirm the final size with the manufacturer's manual and sizing chart, or a professional Manual J load calculation.

Standard sizes: BTU and tons

Mini splits come in standard capacities of 6,000, 9,000, 12,000, 18,000, 24,000 and 36,000 BTU. One ton equals 12,000 BTU, so those are 0.5, 0.75, 1, 1.5, 2 and 3 tons. Not sure which tier your room needs? The mini split calculator applies the factors for you.

Worked example: a 400 sq ft room

A 400 sq ft living room with moderate sun in a hot climate: 400 × 20 = 8,000 BTU, and with the hot-climate factor 8,000 × 1.25 ≈ 10,000 BTU. Round up to the 12,000 BTU (1 ton) head unit — adding moderate sun (×1.1, ≈ 11,000 BTU) lands on the same 1-ton unit. The same room shaded and ordinary stays near 8,000–9,000 BTU, so the 9,000 BTU (0.75 ton) unit is enough.

Sizing tips that get the size right

• Seal and insulate before you size — the 20 BTU/sq ft rule assumes standard insulation; leaky windows and a drafty attic push real load higher, so fix those first. An oversized head short-cycles, cools unevenly and wastes energy.
• Multiple rooms? Use a multi-split — one outdoor unit feeds several indoor heads, each sized to its own room. Don't add separate rooms onto one head unless they're truly open to each other.
• Leave the outdoor unit room to breathe — keep the clearance in the manual on every side; a boxed-in unit runs hot and inefficient.

Sizing a whole house or checking the insulation first? See our guides on what size air conditioner you need and what R-value you need.

Calculate your mini split size →