Knitting Needle Size Chart

A completely separate scale from crochet hooks

Knitting needles and crochet hooks are sized on entirely independent systems, even though both eventually get measured in millimeters. A US size 7 knitting needle (4.5mm) and a crochet hook labeled "7" (also 4.5mm, coincidentally) look like they should match just from the number alone, but that's a coincidence at this one size, not a rule — don't assume a needle and hook with the same printed number are the same diameter without checking the millimeter value on both. When following a pattern from a different country, the metric size is the only figure guaranteed to mean the same thing everywhere.

Why US needle numbers seem to skip around

The US sizing scale isn't evenly spaced in millimeters — the jump from a US 10 to a US 10.5 is only half a millimeter, while the jump from a US 15 to a US 17 is nearly three millimeters. This is a historical artifact of how the scale was built up over time around commonly available diameters, not a mathematical formula, which is exactly why a simple conversion equation can't replace a lookup table for needle sizes.

Checking an unmarked needle

Vintage and well-loved needles often have the printed size worn away. A needle gauge tool with graduated holes is the reliable way to check — slide the needle tip into successively smaller holes until it stops fitting through, and that's the true size, regardless of what a faded stamp might have once said.