Crochet Hook Sizes Decoded: What a 3.5 mm, I-9 or "Size 6" Hook Really Means
Short answer: Stick to the millimeter measurements—they're the only universal sizing rule. Craft brands love using letters, numbers, and regional scales that conflict with one another (which is why a "size 6" could mean 4 mm, 5 mm, or 1.8 mm depending on the hook's origin and material). When in doubt, ignore the label name and match the mm listed on your hook or pattern.
Most hook charts answer one question: “I have this yarn, so which hook do I need?” This guide flips that on its head. Whether you’re staring at an unlabeled hook from the bottom of your craft drawer, trying to decipher an unfamiliar size in a vintage pattern, or sorting through a mystery bundle from a friend, we’ve got you covered. Here’s how to identify what you’re holding—and exactly what to make with it.
If you are starting from a ball of yarn instead, head over to our yarn weight to hook size guide and return here whenever a hook's printed size doesn't match your project.
Three sizing systems, one hook
US letter and number. American hooks pair a letter with a number: B-1, E-4, H-8, J-10. The letter and number climb together, so a later letter is always a bigger hook. Some sizes, like 4.5 mm, have a number (7) but no letter at all.
Old UK and Canadian numbers. These came from the old wire gauge, so the scale is reversed: the bigger the number, the smaller the hook. A UK 9 is 3.5 mm, and a UK 6 is 5 mm. Modern UK hooks are sold in millimeters, but vintage hooks and older patterns still use these numbers.
Steel thread hooks. These are the tiny hooks for crochet thread and lace, and they are also reversed: a steel 14 is about 0.75 mm, a steel 10 is about 1.3 mm. They are sized on their own scale, and brands don't always agree to the tenth of a millimeter.
One Number, Three Completely Different Sizes
Most hook-size mix-ups come down to a single number that means something different depending on who printed it. Here are the usual suspects, using the Craft Yarn Council's standard sizes for the US and steel columns.
| If a hook or pattern says… | US hook | Old UK/Canadian | Steel thread hook |
|---|---|---|---|
| Size 6 | G-6 = 4 mm | 5 mm | about 1.8 mm |
| Size 7 | 7 = 4.5 mm | 4.5 mm (the one size where they agree) | about 1.65 mm |
| Size 8 | H-8 = 5 mm | 4 mm | about 1.5 mm |
| Size 9 | I-9 = 5.5 mm | 3.5 mm | about 1.4 mm |
| Size 10 | J-10 = 6 mm | 3.25 mm | about 1.3 mm |
Watch out for two common label traps: "Size 6" yarn isn't a hook size—it’s the Craft Yarn Council’s code for Super Bulky yarn, which needs a 9 mm or larger hook. Similarly, UK "4 ply" yarn describes a fine yarn (similar to US fingering/sport, calling for a 3–3.5 mm hook), which is completely different from a US "Weight 4" worsted yarn (which calls for a 5–5.5 mm hook). Fortunately, context usually clears things up: modern US patterns use US terms, vintage British or Canadian guides use old UK scales, and thread projects always rely on steel hook sizing.
Letters vs. Millimeters
Millimeter measurements are always more trustworthy than letter sizes, which vary wildly between brands:
- Letter K: Brands list it as 6.5 mm (the Craft Yarn Council standard) and 7 mm (which actually lacks an official letter).
- Letter C: Officially 2.75 mm, though brands like Clover frequently label 3 mm hooks as "C".
- 4.5 mm / Size 7: Officially just a number, yet companies like Addi and KnitPro label it as "G" (which belongs to 4 mm).
- Size I-9: Standardized at 5.5 mm, but occasionally marked as 5.25 mm on older tools.
A quarter-millimeter discrepancy may look trivial, but over a 200-stitch row, it adds up to whole inches.
Pro tip: If a pattern only provides a letter size, look up its official mm equivalent—and when in doubt, use a gauge swatch as your tiebreaker.
Hook by Hook: Sizes and Best Uses
Here is a breakdown of common hook sizes from smallest to largest, including direct links to available hooks on Yarn.com.
0.75 to 1.25 mm (steel 14 to roughly 9)
Best for crochet thread, doilies, lace edgings, and tatting-style trims. These hooks feel more like a sewing tool than a yarn hook, and a good grip matters more here than at any other size. The Clover Soft Touch Steel hook comes in 0.75 mm, 1 mm, and 1.25 mm, with a cushioned handle.
2 to 2.75 mm (B-1 to C-2)
Generally used for lace weight and fine fingering yarns to make shawls, sock-weight colorwork, and very small amigurumi. A 2 mm hook has no US letter, which is why patterns at this size usually just say "2 mm". The Deramores Standard Hook comes in 2, 2.5, and 3 mm.
3 to 3.5 mm (C to E-4)
Your go-to for fingering, sport, and UK 4 ply yarns. The sweet spot for amigurumi—sizing down to a 3 or 3.5 mm hook keeps stitches tight so fiberfill won't peek through (a favorite technique among our team!). A 3.5 mm is a US E-4 and an old UK 9. The Lykke Driftwood 6" hook comes in 3.25 mm (D-3).
3.75 to 4.5 mm (F-5 to 7)
A great choice for lightweight garments, baby blankets, and lacy shawls using sport to DK yarn. Includes the popular 4 mm (G-6) as well as 4.5 mm (Size 7), which is one of the few sizes without an official letter designation.
5 to 5.5 mm (H-8 and I-9)
Best for Worsted and Aran weight yarns—the ideal starting range for beginners. The H-8 suits a firmer fabric for bags and dishcloths, while the I-9 suits blankets and drapier garments. The Deramores Standard Hook and the Lykke Driftwood 6" hook both come in 5.5 mm.
6 to 7 mm (J-10 to K)
When you want fast results, this size range pairs beautifully with heavy worsted to bulky yarns. A 6.5 mm is the official K-10½. The Clover Amour comes in 6.5 and 7 mm, and the Deramores and Lykke hooks both come in 7 mm.
8 to 10 mm (L-11 to N/P-15)
These sizes handly bulky and super bulky yarns for chunky throws, quick beanies, and sturdy baskets. The M/N-13 (9 mm) is the usual starting hook for super bulky ("size 6") yarn. Deramores makes an 8 mm, and Lykke a 10 mm.
11.5 mm and up (P-16 and jumbo)
These sizes tackle jumbo yarn, roving, rugs, or multi-strand crocheting. Letters drop away at this scale, so rely strictly on mm markings, such as those on the Lykke hook (11.5 and 12.75 mm) and the Clover Amour jumbo hook (15 mm).
Unlabeled Hook? Measure the Shaft
Hook markings wear off over time—especially on wooden and bamboo tools. To find your true size, measure the shaft (the smooth barrel right below the hook's throat where your active stitches sit), not the head or the handle.
- Use a Needle Gauge: Slide the hook's shaft into a standard knitting needle gauge until it fits snugly without forcing it. The matching hole is your millimeter size.
- Compare Side-by-Side: Hold your unlabeled hook directly against a labeled hook to visually spot the match.
- Work a Test Swatch: Stitch a quick 10-stitch gauge swatch and compare it against one made with a known hook size to confirm your fabric density.
Pro tip: once you have measured a mystery hook, write the mm size on a bit of washi tape around the handle. You will thank yourself the next time the pattern asks for it.
Related reading
- What size crochet hook for each yarn weight
- The complete crochet hook buying guide
- The best crochet hooks we have tested
- Crochet hook size chart
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FAQ
What is a 3.5 mm crochet hook in US size?
An E-4. In the old UK/Canadian system it is a 9. It suits sport weight and UK 4 ply yarn, and it is the most common hook for cotton amigurumi worked in DK.
What is a size 6 crochet hook?
It depends on where the 6 came from. On a US hook it is a G-6, 4 mm. On an older UK or Canadian hook it is 5 mm. On a steel thread hook it is about 1.8 mm. Look for the millimeter size before you buy or start.
What size crochet hook do I use for 4 ply yarn?
UK 4 ply is roughly US fingering or light sport weight, so start with a 3 to 3.5 mm hook (C to E-4). Don't confuse it with weight 4 on a US label, which is worsted and wants a 5 to 5.5 mm hook.
What is an I-9 crochet hook in mm?
Most brands make the I-9 at 5.5 mm. A few older charts and hooks list it at 5.25 mm, so if a pattern's gauge is tight, check which one you own.
What is the most common crochet hook size?
5 mm (H-8) and 5.5 mm (I-9) are the sizes most worsted yarn labels and beginner patterns call for, which makes them the most-used hooks in most kits.
What size crochet hook do I need for size 6 yarn?
Size 6 yarn is super bulky, so start at 9 mm (M/N-13) and go up to 15 mm or more for a looser fabric. Don't mix this up with a size 6 hook, which is a much smaller tool.
Crochet Hook Sizes Decoded: What a 3.5 mm, I-9 or "Size 6" Hook Really Means
You're holding a hook or reading a pattern and the size doesn't make sense. Here's what...
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