What This Tool Does
NeedleSizeConverter.com is a free, fast converter for both knitting needle sizes and crochet hook sizes. Toggle between the two on the home page, enter any size in your system of choice, and instantly see the equivalent in every other system — no chart-flipping required.
For knitting needles, the converter covers US, UK, metric (mm), and Japanese sizes from 2.0 mm up to 25.0 mm. For crochet hooks, it covers US letter sizes (B/1 through S) and UK/Canada numbers from 2.0 mm up to 19.0 mm. Both tools include a complete printable reference chart with yarn weight recommendations for each size.
Why Sizing Systems Are So Confusing
Knitting and crochet patterns are written all over the world, and the sizing systems used in the US, UK, and Japan developed independently of each other. The result is that the same physical needle — say, a 4.0 mm needle — is labelled "US 6" in American patterns, "UK 8" in British ones, and has no direct Japanese equivalent. UK and US numbering run in opposite directions: larger US numbers mean larger needles, while larger UK numbers historically meant smaller ones.
Crochet hooks have the same problem. US hooks use a letter-and-number system (B/1 through S) while UK and Canadian hooks use a descending number system that shares no logical relationship with the US labels. A US G/6 hook is 4.0 mm; a UK 8 hook is also 4.0 mm, but there's no intuitive way to know that without a chart. The metric measurement is the only label that means the same thing everywhere.
Data Accuracy
Knitting needle data is based on the Craft Yarn Council (CYC) standard for US sizes, the traditional British Imperial system for UK sizes, and the JIS (Japanese Industrial Standard) for Japanese sizes. Crochet hook data follows the CYC standard for US letter sizes and the traditional UK/Canada hook numbering system. Where no direct equivalent exists in a given system, the converter shows the nearest standard size and marks it as approximate.
Always measure your needle or hook with a gauge tool before starting a project, slight manufacturing variations exist between brands.
About the Author
"I got tired of Googling needle sizes mid-project, so I built the tool I wished existed." — Emma R.
NeedleSizeConverter.com was created by Emma R., a knitter with over a decade of experience and a background in web development. The tool started as a personal fix for a recurring problem: mid-project, faced with a vintage pattern using UK needle numbers that didn't match anything on a modern label, with no quick way to look them up.
More recently, Emma has passed the craft on to her son. They work on hats and scarves together, which, if anything, has only reinforced how often you need a quick needle size lookup mid-project.
Emma's technical background meant the conversion logic could be built precisely rather than approximated, and her experience as a crafter meant she knew what information to surface, and what to leave out.
About the Site
NeedleSizeConverter.com is operated by Emma R. If you have questions, corrections, or feedback, please reach out at emma@needlesizeconverter.com.
The site is intentionally lightweight, no accounts, no tracking beyond anonymous analytics, no paywalls. Just a useful tool, free for every knitter and crocheter.