Crochet Hook Size Conversion
This crochet hook size conversion chart matches metric, US, and old UK hook sizes so you can follow any pattern. Pick a millimeter size and see its US letter or number and its UK equivalent at a glance.
US, UK and metric hook sizes
How the crochet hook size conversion works
Crochet hooks are labeled in three overlapping systems. Metric sizing states the hook diameter in millimeters and is the most precise and universal. The US system uses letters and numbers, such as H-8, while the older UK system runs the opposite way, with larger numbers meaning smaller hooks. This crochet hook size conversion lines them up side by side.
Because letter and number labels are not perfectly standardized between brands, the millimeter measurement is the one to trust. When a pattern and a hook disagree, match the millimeters. If a hook is unmarked, measure its shaft with a gauge ruler or a caliper and read across to the other systems.
Worked example
Say a US pattern calls for an H-8 hook and you only have metric hooks. Looking up H-8 in the conversion, it equals 5.0 millimeters, which is one of the most common crochet hook sizes.
That same 5.0 millimeter hook is a size 6 in the old UK system. So a British vintage pattern asking for a number 6 hook and an American pattern asking for an H-8 are calling for exactly the same tool.
Reading a crochet hook size conversion correctly
The metric and US columns here follow the Craft Yarn Council hooks and needles chart, the industry standard in North America. Keep in mind that some hooks carry both a letter and a number, like K-10.5, and manufacturers occasionally differ by a fraction of a millimeter, so the mm value is always the tiebreaker.
For very fine thread crochet, a separate steel-hook system applies, where higher numbers mean smaller hooks and sizes run down to under 1 millimeter. If your pattern uses steel hooks and crochet thread, check the dedicated steel hook chart instead, since those numbers do not match the regular hook sizes above.
Related calculators: All craft calculators · WPI Yarn Weight Calculator · Crochet Gauge Calculator
Frequently asked questions
What is an H-8 crochet hook in mm?
An H-8 crochet hook is 5.0 millimeters, one of the most widely used sizes and a common match for worsted-weight yarn. In the old UK system it corresponds to a size 6.
Which crochet hook sizing system is most accurate?
The metric millimeter system, because it states the actual diameter of the hook. US letters and old UK numbers can vary slightly between brands, so when in doubt follow the millimeter measurement.
Why do UK hook numbers go the opposite way?
The old UK and Canadian system numbers hooks by a wire gauge, where larger numbers mean thinner hooks. That is the reverse of metric or US letters, which is why a conversion chart is so useful.
How do I measure an unmarked hook?
Slide the hook shaft into a needle or hook gauge until it fits snugly, or measure the shaft with calipers in millimeters. Then read across the conversion to find the US letter and UK number.
Are steel crochet hooks sized the same way?
No. Steel hooks used for thread crochet have their own scale where higher numbers mean smaller hooks, down below one millimeter. Use a dedicated steel hook chart rather than the standard hook conversion for those.