Crochet Hook Size Converter
Convert between US letter, US number, metric (mm), and UK/Canadian crochet hook sizes. Enter any size to instantly find all equivalents.
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Full Conversion Table
| US Letter | Metric (mm) | UK / Canadian | US Number | Recommended Yarn |
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About Crochet Hook Sizes
Crochet hook sizing uses four different systems worldwide. The US letter system (B through P and beyond) is common in North American patterns. The metric system specifies the hook diameter in millimeters and is the international standard. The UK/Canadian system uses numbers on a reverse scale — a higher number means a smaller hook.
Hook sizes are not perfectly standardized across manufacturers. Even within the same nominal size, hooks from different brands may vary by 0.1–0.25mm. When precision matters, measure your hook with a mm gauge tool.
Always work a gauge swatch when substituting hook sizes. The recommended yarn weight for each hook size is a guideline — many crocheters intentionally use larger or smaller hooks to achieve specific textures or effects.