Hook Size Calculator — Metric, US & UK Crochet Hook Conversion

Metric sizes are the hook diameter in mm. US sizes use letters (B through S) and numbers. UK old sizes run in reverse (highest number = smallest hook). The metric size is the most precise and universally understood.

The Craft Yarn Council recommends a 4.5-5.5mm hook for DK (Light, category 3) yarn, with 5.0mm being the most common starting point — adjust to suit your desired fabric density. For amigurumi, go down 1-2 sizes for tighter stitches. For lacy crochet, go up 1-2 sizes for a more open fabric. Always check the yarn ball band for the suggested hook size.

The metric measurements are the same (a 4mm hook is the same diameter as a 4mm needle). However, US/UK numbering systems are completely different between hooks and needles. Always use the metric size for accurate comparison.

Inline hooks (like Susan Bates) have a uniform shaft and pointed tip. Tapered hooks (like Boye) have a rounded tip and throat that narrows. The choice is personal preference; neither affects the size conversion, only how the yarn feeds onto the hook.

US patterns list the US letter or number size; UK patterns list the metric size. They refer to the same physical hook. Always verify by checking the metric (mm) size on your hook. This converter helps you match between systems.