What does the letter on a crochet hook mean?

US hook sizes use a letter and number together, like H-8. The letter is the US sizing system; the number is roughly the metric size in millimetres rounded down. UK and old sizing use different numbers entirely, which is why checking the millimetre size is the safest way to compare hooks.

Is a bigger hook number a bigger hook?

In millimetre and US letter sizing, yes, a higher number means a larger hook. In UK and old imperial sizing it's reversed, a smaller number means a larger hook, which is the most common source of confusion when reading older patterns.

Why do my stitches look different from the pattern photo even with the right hook size?

Hook size alone doesn't guarantee matching gauge. Your personal tension, the yarn brand, and even the hook material can shift your stitch size. Always crochet a small gauge swatch before starting a sized project.

Reference

Crochet Hook Sizes: The Complete Conversion Chart

Metric, US, and UK hook sizes side by side, with the yarn weight each one is built for.

Updated 2026 · 4 min read

Crochet hooks are labelled three different ways depending on where they're made: metric millimetres, US letters and numbers, and the older UK system. A pattern written in one system and a hook packet labelled in another is the single most common reason beginners buy the wrong size. This chart lines all three up.

Safest method: always match by millimetre size first. It's the one measurement that means the same thing everywhere, regardless of which country made your hook or wrote your pattern.

Full size conversion chart

Metric (mm)	US size	UK size	Typical yarn weight
2.25mm	B-1	13	Lace / fingering
2.75mm	C-2	12	Fingering
3.25mm	D-3	10	Sport