About the charts
US and metric values follow the Craft Yarn Council Hooks & Needles chart. UK labels are traditional approximations FiberCalc lists for convenience—CYC does not publish UK sizes—so prefer the stamped millimeter marking. Some historical UK sizes do not map 1:1.
Worked example
US 8 knitting needle = 5.0 mm = UK 6. Crochet H-8 = 5.0 mm. CYC lists US 17 at 12.50 mm and 12.75 mm; this converter uses 12.75 mm.
Tips
Metric millimeters are the most reliable cross-brand label. Swap tools if your gauge is off before changing patterns.
Common mistakes
Assuming US crochet letters match knitting US numbers; using lace hooks for jumbo yarn; treating UK labels as CYC standards.
Gauge still rules
Two US 8 needles from different brands can differ slightly; trust stamped millimeters when you have them. If your swatch is dense, go up a metric size; if loose, go down. Steel lace hooks and Tunisian hooks follow related but not identical naming—use metric when mixing tool types.
Limits and how to use the results
Estimates run only in your browser. Prefer a published pattern’s tested yardage when it conflicts with this planner. Density factors and assumptions are documented on the methodology page; legal terms cover planning-use limits.
Project pairing tips
Match needle or hook size to both yarn category and desired fabric: a DK yarn on 4 mm needles makes a different cloth than the same yarn on 3.5 mm. Use this converter when a pattern lists only US or only UK sizes, then confirm with a metric gauge swatch. Tunisian crochet hooks and double-pointed needles still follow millimeter sizing even when brand names differ. Store a small tape measure in your notions pouch to verify unmarked thrifted tools.
FAQ
What size is a US 8 needle in mm?
5.0 mm (UK 6).
What is an H crochet hook in mm?
H-8 is 5.0 mm on the CYC chart.