Walking into a craft shop and facing a wall of hooks labelled with letters, numbers, and millimetres can be overwhelming. Let's demystify crochet hook sizing so you always pick the right tool for your yarn.
The three sizing systems
US letter sizes (B through S) are the most common in patterns. Metric millimetre sizes are the universal standard — the actual diameter of the hook shaft. UK/Canadian numbers are an older system still found in vintage patterns. Always check the mm size if you are unsure.
Yarn weight and hook pairs
Lace weight (0-1 ply) uses a 2.0-3.5mm hook. Fingering/sock weight (super fine) works with 3.5-4.5mm. DK/light worsted (8 ply) pairs with 4.5-5.5mm. Worsted/aran (10 ply) takes 5.5-6.5mm. Chunky/bulky (12-14 ply) needs 6.5-9mm. Super bulky uses 9mm and up.
Your yarn label always recommends a hook size. Start there, then adjust up or down based on your personal tension.
The most important rule is to make a gauge swatch before starting any project that needs to fit — garments, hats, amigurumi. A 10x10cm swatch in the pattern stitch will tell you if your hook-yarn combo produces the right fabric density.