Crochet Hook Sizes: Check Millimetres Before Letters and Numbers
Choose a crochet hook using its millimetre size, handle unclear letter labels, and keep a project record that is easy to return to.
You have a crochet hook labelled with a letter, another with a number and a pattern that specifies millimetres. Start with the mm measurement on the hook’s packaging rather than assuming that a letter always identifies one size.
Craft Yarn Council explains that manufacturers’ letter and number labels vary. The metric size gives you a measurement to compare.
Find the millimetre marking
- Read the hook-size requirement in your pattern.
- Find the mm size on the packaging; record any mm marking on the hook too.
- Compare the measurements before relying on the letter or number.
Original shopping example: your practice pattern specifies a 4.0 mm hook. Two products have different letter labels, but both packages state 4.0 mm. The package measurement is the size reference to compare. That does not establish that every detail of the hooks, or the fabric you make with them, will be identical.
Handle an unclear hook label
If you only have a letter or number, treat the metric size as unconfirmed. Look for the original packaging or the manufacturer’s listing for that exact product. A generic conversion chart may help you investigate, but do not record a guessed measurement as a confirmed one.
If the packaging and hook marking appear to disagree, check with the maker or seller. You can also choose a clearly labelled hook for a new project rather than carrying an unresolved size into your notes.
This is a suggested checking sequence, not a universal conversion table. It deliberately avoids assigning one fixed mm size to every letter.
Check the fabric with a swatch
Matching the stated hook size is not the end of the check. Make a gauge swatch and compare it with the pattern’s gauge instructions. Use the specified yarn and sample stitch pattern so you are testing the intended sample.
When the measured sample does not match, a different hook may be the next trial. Our crochet gauge guide explains the direction to try for more or fewer stitches over the same length, with an arithmetic example.
Leave a useful project record
A small note makes a later restart less ambiguous. Suggested fields are:
- Hook: confirmed mm size, plus maker or model if known.
- Label: any letter or number printed on that particular hook.
- Pattern: title and where the gauge instructions appear.
- Trial: sample stitch, measurement length, stitch count and row count.
- Decision: which hook you used after checking the sample.
A blank note template might read: “Hook: ___ mm; maker/model: ___; printed label: ___; gauge result: ___; next step: ___.” It is a suggested workflow, not a record of a real customer’s project.
Keep the terminology note nearby as well. If your pattern uses unfamiliar stitch names, start with the US/UK crochet terminology check before making the sample.
Keep your project notes together
Paper notes work well. If you prefer a phone, our StitchCount row counter and project notes app includes project notes alongside row and repeat counters. Choose whichever method helps you return to your own pattern with a clear next step.
Sources and scope
Craft Yarn Council: Hooks & Needles, checked October 18, 2026. AppMaven wrote this guide and its examples; the source organization has not endorsed this article. No source diagrams or complete patterns are reproduced.
This is a reference-based explanation, not a hands-on test of a particular yarn, hook or pattern.