Line set size calculator
Tell us your mini-split's size and how far apart the units are. We'll give you the exact line set diameter and length to buy. Know your brand but not the BTU? Look it up by brand →
How to size a mini-split line set
A line set has two copper tubes — a small liquid line and a larger suction (vapor) line — wrapped in insulation. Two things decide what you need:
- Diameter is set by your system's capacity. The manufacturer engineers each unit for a specific pair of line diameters, so this is not a choice — it's a spec to match.
- Length is set by the distance between your indoor head and outdoor condenser. Measure the real routing path, add slack, and round up to the next stocked length.
Typical line set size by mini-split capacity
| Mini-split size | Typical line set (liquid × suction) |
|---|---|
| 9,000 BTU | 1/4" × 3/8" |
| 12,000 BTU | 1/4" × 1/2" |
| 18,000 BTU | 1/4" × 1/2" |
| 24,000 BTU | 3/8" × 5/8" |
| 36,000 BTU | 3/8" × 5/8" |
| 48,000 BTU | 3/8" × 5/8" (some 3/8" × 3/4") |
These are the most common pairings, but they vary by brand. The single source of truth is your unit's installation manual — it lists the exact liquid and suction line sizes and the maximum line length. See the full line set size chart.
A few rules of thumb
- Don't oversize or undersize the diameter — the lines are matched to the compressor for correct refrigerant flow and oil return.
- Round length up, never down. Coiling a little extra is fine; stretching a short line set is not.
- On pre-charged DIY systems, watch the maximum pre-charged length (often 25 ft). Longer runs may need refrigerant added by a pro.
Common questions
What size line set do I need for a mini-split?
Diameter is set by your mini-split's BTU rating; length by the distance between the indoor and outdoor units. A 9,000 BTU system typically uses 1/4" × 3/8"; 12,000–18,000 BTU uses 1/4" × 1/2"; 24,000–36,000 BTU uses 3/8" × 5/8". Always confirm against your unit's installation manual.
Can I use a bigger line set than recommended?
No — match the manufacturer's specified diameters. The wrong line size changes refrigerant velocity and oil return, which hurts performance and can void the warranty.
How long can a mini-split line set be?
Many systems allow runs up to about 50 feet, but pre-charged DIY units often have a maximum line length (frequently 25 feet) before you must add refrigerant. Check your unit's manual for its maximum line length and pre-charge limit.
This calculator provides general guidance based on common mini-split line set sizing. It is not a substitute for your unit's installation manual or professional advice. Always verify line diameters, maximum line length, and pre-charge limits with the manufacturer before purchasing or installing.