Mini Split Sizing Calculator – What Size Do I Need? | CalcLoft
What size mini split do I need? Estimate your cooling load in BTU from room area, ceiling height, sunlight and climate — then get the right head size in seconds.
How to size a mini split
Mini split (ductless) systems are sized in British Thermal Units per hour (BTU/hr). A common starting point is 20 BTU per square foot, then multiply by correction factors for ceiling height, sunlight and climate:
BTU ≈ room area (sq ft) × 20 × (ceiling height ÷ 8) × sunlight factor × climate factor
Apply these adjustment factors: Sunlight — Low ×1.0, Moderate ×1.1, High ×1.25 · Climate — Mild ×0.9, Moderate ×1.0, Hot ×1.25 · Ceiling — divide the actual ceiling height by 8 (so a 10 ft ceiling multiplies the load by 1.25). Add 500 BTU per person beyond the first two.
Example: a 400 sq ft room with 8 ft ceilings, moderate sunlight and a moderate climate works out to 400 × 20 × 1.0 × 1.1 × 1.0 = 8,800 BTU, which rounds up to a 9,000 BTU (0.75 ton) head. The result is an estimate — for an exact number, ask for a professional load calculation (Manual J) before buying.
Mini split size by room area
The table below shows the area × 20 baseline load for common room sizes, plus the recommended head after applying a ×1.25 hot-climate factor and rounding up to a standard head size. Rows assume 8 ft ceilings and an ordinary closed room; use the calculator above to fine-tune for sunlight, ceiling height and occupants.
| Room area (sq ft) | Base load (area × 20 BTU) | Hot climate suggestion (×1.25, rounded up) |
|---|---|---|
| 200 | 4,000 BTU | 6,000 BTU (0.5 ton) |
| 300 | 6,000 BTU | 9,000 BTU (0.75 ton) |
| 400 | 8,000 BTU | 12,000 BTU (1 ton) |
| 500 | 10,000 BTU | 18,000 BTU (1.5 ton) |
| 600 | 12,000 BTU | 18,000 BTU (1.5 ton) |
| 800 | 16,000 BTU | 24,000 BTU (2 ton) |
What does "ton" mean on a mini split?
Cooling capacity is often quoted in tons: 1 ton of cooling = 12,000 BTU/hr. So a 0.75 ton head is 9,000 BTU, a 1.5 ton head is 18,000 BTU and a 2 ton head is 24,000 BTU. Single-zone heads are commonly sold in 6,000 / 9,000 / 12,000 / 18,000 / 24,000 BTU sizes (0.5–2 ton). When your calculated load falls between two sizes, always choose the next size up — an undersized unit runs constantly and never keeps up.
Quick mini split sizing tips
- 12,000 BTU ≈ 1 ton — 9,000 BTU is 0.75 ton, 18,000 BTU is 1.5 ton, and so on.
- Multiple rooms? Use a multi-zone system. One outdoor unit can feed several indoor heads (typically 2–5, depending on the model) so each room gets its own thermostat.
- Seal and insulate first. A tight, insulated room needs far fewer BTUs, so a smaller (and cheaper) head will do the job.
Related guides
Mini split sizing FAQ
What size mini split do I need for a 400 sq ft room?
Most installers quote around a 12,000 BTU (1 ton) head for a typical 400 sq ft room, because the price jump from a 9,000 BTU head is small and it covers sunny or hot rooms. At the default settings above (8 ft ceilings, moderate sunlight, moderate climate), 400 sq ft works out to about 8,800 BTU — so a 9,000 BTU (0.75 ton) head is enough in a well-shaded, average room, and 12,000 BTU gives you headroom for bright or hot conditions.
How many BTU is a ton of cooling?
One ton of air conditioning equals 12,000 BTU per hour. Mini split heads are sized in steps of roughly 6,000 BTU: 0.75 ton = 9,000 BTU, 1 ton = 12,000 BTU, 1.5 ton = 18,000 BTU and 2 ton = 24,000 BTU.
How many indoor units can one outdoor unit run?
Usually 2–5 indoor heads (zones) per outdoor unit, depending on the model and the outdoor unit's capacity. Each head is sized for its own room, and the outdoor unit must be large enough to cover the combined load — check the manufacturer's spec sheet for the maximum number of zones.
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