Convert knitting needle and crochet hook sizes between US, UK/Canadian and metric millimeters with instant lookup tables.
Crochet-hook lettering varies by brand — trust the mm stamped on your hook. Charts give a starting size; always swatch to confirm gauge.
About this tool
Knitting Needle & Crochet Hook Size Converter is a free, in-browser tool that translates needle and hook sizes across the three systems knitters and crocheters run into: US, UK/Canadian and metric millimeters. Choose needles or hooks, pick the system you have, type your size, and it returns the equivalents in every system at once. Everything is looked up locally — nothing is uploaded.
The method uses standard bidirectional conversion tables built from the published equivalents. Metric millimeters are the anchor, since they are unambiguous, with US and UK/Canadian sizes matched to each millimeter step. A US 8 knitting needle, for example, is 5.0 mm and UK 6. When you enter a millimeter value the tool snaps to the nearest listed size; when you enter a US or UK size it matches the size code directly, including hook letters and numbers such as H/8 or K/10½.
Use it when a pattern lists a size in a system your needles or hooks are not marked in. Because crochet-hook lettering is inconsistent between brands, the hook table shows the common combined letter-and-number labels — always double-check against the size printed on your actual hook, and knit or crochet a gauge swatch when substituting.
Frequently asked questions
What is a US 8 knitting needle in mm?
A US size 8 knitting needle is 5.0 mm, which is UK/Canadian size 6. Enter 8 with the US system selected and needles chosen, and the tool shows the metric and UK equivalents side by side.
Why are crochet hook sizes so inconsistent?
US crochet hooks use both letters and numbers, and the letter-to-millimeter mapping varies between manufacturers. The tool lists the common combined labels (such as H/8), but you should always trust the millimeter size stamped on your specific hook.
Can I convert from a millimeter size?
Yes. Select the mm system and enter the millimeter value; the tool snaps to the nearest standard size and shows the US and UK equivalents. This is handy for unmarked or imported needles and hooks labeled only in metric.
Does needle size guarantee my gauge?
No. Size charts get you into the right range, but your gauge depends on your yarn and your tension. Always work a gauge swatch and change needle or hook size up or down until you match the pattern's stitch and row counts.