Crochet Hook Sizes — US to Metric Conversion

US hook sizes use a letter-and-number system (B-1, C-2, G-6, etc.) that doesn't correspond directly to millimeter measurements in a simple way. A G hook is 4.0mm but an H hook jumps to 5.0mm — there are gaps and inconsistencies in the letter sequence. Metric sizes are the more reliable standard: if you're buying hooks online or internationally, always check the millimeter size rather than the letter designation.

Hook sizes from different manufacturers may vary slightly even at the 'same' size. A 4.0mm hook from one brand may work slightly differently than a 4.0mm from another due to throat shape, taper, and metal vs. wood construction. If gauge is critical for a fitted garment, always swatch with the actual hook you'll be using.

Steel crochet hooks (for fine thread lace work) use a different numbering system that runs in reverse — larger numbers are smaller hooks. Steel hook sizes are not included in the standard lettered system and range from size 00 (3.5mm) down to size 14 (0.75mm). They're used for crochet thread, doilies, and fine lace — not for standard yarn.

Most yarn labels show a recommended hook size range on the label. This is the starting point, not a rule — your personal tension (how tightly or loosely you crochet) affects whether you should go up or down a size. If your swatch is too small (your stitches are tighter than the pattern gauge), use a larger hook. If your swatch is too large, use a smaller hook.

For colorwork patterns like graphghans and tapestry crochet, go down one hook size from the yarn label recommendation. Colorwork tends to produce looser stitches because you're not pulling yarn as tightly with each stitch, and a tighter gauge keeps the carried yarn from showing through. For amigurumi and toys, go down significantly — you want a very tight fabric so stuffing doesn't show through gaps.