Crochet Hook Conversion Calculator
US letter sizes to metric and back.
Convert crochet hook sizes between US letter and number sizes and metric, with the yarn weight each suits.
US hook size
H/8
5 mm
Above L the letters stop being reliable: a 9 mm hook is sold as M, N or 13 depending on the manufacturer. Buy by millimetres at that end of the range.
How the Crochet Hook Conversion Calculator works
US crochet hooks carry a letter and a number that mean the same thing — an H is an 8, a J is a 10 — while the rest of the world uses millimetres. The letters run out at the large end and start doubling up, which is why the biggest hooks appear as M/N/13 and N/P/15 depending on the manufacturer, and why the millimetre figure is the one to trust.
Also known as: hook size chart · crochet hook size converter
Letters, numbers and millimetres
US crochet hooks carry both a letter and a number that mean the same thing: H and 8 are the same 5 mm hook, J and 10 are the same 6 mm. The dual labelling is historical and the two are used interchangeably in patterns.
The letters were extended past L inconsistently by different manufacturers, which is why a 9 mm hook appears as M, N or 13 depending on the brand, and a 10 mm as N or P. At that end the letter is not a reliable identifier.
Millimetres are unambiguous throughout, which is why patterns from outside the US give them and why they are the sensible thing to buy by for anything above about 8 mm.
A worked example
A pattern calls for an H hook. That is 5 mm, the most common size for worsted weight yarn, and the size most beginner patterns assume.
A pattern calling for an N hook is less clear: it could be 9 mm or 10 mm depending on who made the hook. Checking the millimetre marking on the hook itself is the only way to be sure, and if the pattern gives a gauge, that settles it.
Where the answer misleads
Hook size sets a starting point and not much more. Crochet gauge varies with how tightly the yarn is held and how far into the stitch the hook is inserted, and both vary more between crocheters than needle technique does between knitters. Two people with the same hook and yarn can differ by half a stitch per inch.
Hook head shape also matters. Inline hooks and tapered hooks form slightly different stitch sizes at the same nominal diameter, which is why some crocheters find they need a different size when switching brands.
Finally, the recommended hook on a ball band is a suggestion for a fabric the manufacturer liked. For amigurumi you would go down a size or two to make a dense fabric that stuffing cannot show through; for a drapey shawl, up.
Frequently asked questions
What size is an H hook?
5 mm, also written as H/8. It is the most commonly used hook size for worsted weight yarn and the one most beginner patterns assume.
Why do large hooks have two letters?
Because the letter series was extended inconsistently by different manufacturers once it ran past L. A 9 mm hook is sold as M, N or 13 depending on the brand, so buying by letter alone at that end is unreliable.
Are crochet hooks and knitting needles the same sizes?
The metric sizes overlap but the scales do not. A 5 mm hook and a 5 mm needle are the same diameter, but a US 8 needle is 5 mm while a US 8 hook is also 5 mm only by coincidence of numbering — the letter is the reliable US hook designation.
Does hook size alone set my gauge?
No. Crochet gauge varies with how tightly you hold the yarn and how far into the stitch you insert the hook, at least as much as with hook size. Swatch rather than assuming the recommended hook gives the stated gauge.