Crochet Hook Size Chart: Letter, Number, mm and UK Conversions

An H-8 crochet hook is 5 mm. It is one of the most popular sizes, suited to medium weight (worsted) yarn for blankets, hats and amigurumi with a tighter tension. UK patterns call it a size 6 in the old numbering, but most now use millimeters.

A 5 mm (H-8) or 5.5 mm (I-9) hook with smooth medium weight yarn is easiest for beginners. The stitches are easy to see, the hook is comfortable to hold, and projects grow quickly. An ergonomic handle helps if your hands tire or ache.

US hook sizing grew from separate letter and number systems, and not every millimeter size has both. Some sizes such as 3.25 mm D-3 or 6.5 mm K-10.5 carry both, while others like 7 mm have no US letter. The millimeter size printed on the hook is always the most reliable.

Steel hooks are for crochet thread: doilies, lace, edgings and fine motifs. They use their own scale where a higher number is a smaller hook, from about 3.5 mm down to under 1 mm. Brands differ slightly, so the chart lists alternate millimeter sizes where published charts disagree.

Check the yarn label first; it lists a recommended hook. As a guide, fine yarn uses 3.5 to 4.5 mm, medium yarn 5 to 6.5 mm, bulky yarn 6.5 to 9 mm and super bulky 9 mm and up. Go down a size for amigurumi, which needs tight stitches that hide the stuffing.