Crochet hook sizes are usually expressed in millimetres, but you may also see letters or numbers. The millimetre measurement is a useful reference because lettering systems vary.
What the millimetres mean
The printed millimetre measurement describes the hook size. Bigger hooks generally produce larger loops when the same yarn and tension are used, and smaller hooks produce smaller loops.
Start with the pattern recommendation
Patterns normally recommend a hook and yarn combination. Treat that as your starting point, not a guarantee: individual tension varies.
Check your yarn label
Many yarn labels include a suggested hook range. If you are following a garment or fitted pattern, the pattern's gauge is especially important.
What is gauge?
Gauge describes how many stitches and rows fit in a measured area. A small swatch helps you see whether your fabric will match the planned size.
Amigurumi can be different
Stuffed toys often use a smaller hook for dense fabric so the stuffing is less visible. If your stitches have holes, see Why Does My Amigurumi Have Holes?.
A simple buying tip
For a first project, buy the hook size that the tested pattern recommends. Avoid spending money on a large collection until you know the yarn types and projects you enjoy.