Needle & Hook Size Converter

Three different sizing systems are in active use for knitting needles alone — US numbers, metric millimetres, and the older UK/Canada numbering many vintage and British patterns still use — and none of the three map onto each other in an intuitive way. This tool converts between all three for knitting needles, and between US and metric for crochet hooks, using the standard published equivalence tables rather than an approximation.

Why this needs a real table, not a formula

Needle and hook sizing wasn't standardized by a clean mathematical rule — the systems were established independently and then cross-referenced, which is why the spacing between consecutive sizes isn't uniform (the jump from a US 10.5 to a US 11 is 1.5mm, while most smaller jumps are 0.25-0.5mm). A formula that assumed even spacing would misconvert exactly the sizes where the table has a wider or narrower real-world gap. This tool looks up the actual published table every time, for every size, rather than interpolating from a curve.

The one thing that trips people up: UK sizing runs backward

Metric and US sizing both climb together — a US 10 (6mm) is a larger needle than a US 6 (4mm), and the metric figure climbs right alongside the US number. UK/Canada (old) sizing was built on the opposite convention: its number counts down as the physical needle gets thicker. A UK 14 (2mm) is one of the finest needles in common use, while UK 0, then 00, then 000, march upward in digit count while the needle itself keeps getting larger. Assume "smaller printed number, smaller needle" on a vintage British pattern the way US sizing trains you to, and you'll reach for a needle that's wrong by a wide margin, not a near miss.

A few fixed points, so you can sanity-check the tool

US 7 and UK (old) 7 both happen to be 4.5mm — one of the only points on the whole range where the US and UK numbers coincide by coincidence, not by any underlying rule. Don't extend that coincidence to any other size: a US 8 is 5mm and UK 6, not UK 8. At the small end, US 0 is 2mm and UK 14; at the large end, US 15 is 10mm and UK 000. See the full needle conversion table for every published size in one place.

Crochet hooks: US letters, US numbers, and metric

US crochet hooks mix letters and numbers in one continuous scale (B-1 through roughly S), which is its own small source of confusion since the letter and the number both increase together but aren't derived from each other by a fixed formula either — they're simply the two labels the same physical hook has always carried. This tool converts any US hook designation directly to its metric size. Steel hooks, used for fine lace-weight crochet thread, use an entirely separate smaller-number-is-bigger numeric scale not covered by this table, since they're a distinct product category from the standard aluminum/plastic hook range.

When "interpolated" appears in a result

If you enter a metric size that falls between two published table sizes, the converter returns the nearest listed size and flags the result as interpolated rather than silently picking one — so you know the returned US or UK number is an approximation to the nearest standard size, not an exact published equivalence for the number you entered.

FAQ

Why does my needle package say a slightly different metric size than this tool? Some needle manufacturers round their stated metric size slightly differently, or use a size just outside the standard published table. Trust the metric measurement printed directly on your needle over any US/UK label conversion if the two ever seem to disagree.

Is a bigger US needle number always a bigger needle? Yes, consistently, across the entire US range — this is exactly the assumption that fails for UK/Canada (old) sizing, which is why the calculator exists rather than a simple lookup table alone.

Does this converter cover double-pointed and circular needles differently from straight needles? No — needle size (the diameter) is identical across straight, circular, and double-pointed needles of the "same" size; only the needle's length and, for circulars, cable length differ by format, not the size conversion itself.