This has a 2:1 shrink ratio at about 90 degrees C. Polyolefin Tubing.
Instructions: While a heat gun is the proper way to shrink the tubing, if you don’t mind scuffing up your soldering iron, the barrel can be held close by to slowly shrink the tubing, too.
About wire sizes: Below are tables listing AWG gauge wire sizes in addition to the thickness of common insulated versions of those wires. Insulation thicknesses are not standard, so heat shrink tubing that works on one type of 18 gauge wire may not work on another. Also, stranded wire is usually a little thicker. The sizes below are probably on the small side (they’re based on Alpha PVC hookup wire). Pick a tubing size that has a starting size greater than the wire and a shrunk size less than the wire.
AWG Gauge Wire Sizes:
Recommended Tubing Sizes for Various Gauges