Crochet Hook Sizes Decoded: What a 3.5 mm, I-9 or "Size 6" Hook Really Means

You have a hook in your hand, but the pattern calls it something different. Before buying a replacement, look for a second marking on the handle or thumb rest. Many hooks show both a letter or number and a measurement in millimeters. That measurement is the most useful way to compare tools across brands.

This Yarn.com guide starts with the hook you already own. If you are choosing a tool for a ball of yarn instead, use our yarn-weight-to-hook-size guide.

Crochet hook size chart: millimeters to US labels

The chart below follows the Craft Yarn Council hook-size table. Intermediate sizes are included because they are still produced by certain brands and commonly found in vintage collections. Where you see “No US label listed,” it simply means the Craft Yarn Council does not assign a standard letter or number to that metric measurement—not that the hook size doesn't exist.

Read the mm, not just the letter. The chart includes both G at 4.25 mm and G-6 at 4 mm, I at 5.25 mm and I-9 at 5.5 mm, and J at 5.75 mm and J-10 at 6 mm. Q also appears at more than one metric size. A letter alone does not guarantee an exact match.

Why a “size 6” hook can mean different things

US hooks: Letters and numbers often appear together, such as G-6 or H-8. A G-6 is 4 mm in the Council chart. If an older hook is labeled only with a plain "6" without an accompanying letter or metric millimeter mark, look closer—it may belong to a vintage or steel sizing system.

Old UK and Canadian hooks: Older British and Canadian hooks use a reversed numbering scale where larger numbers indicate smaller hooks. Never treat a vintage UK number as a modern US size. Always consult the vintage pattern’s conversion key or the original maker's sizing guide before swapping in a modern hook.

Steel thread hooks: Steel hooks follow their own independent, reversed numbering system for delicate thread work. A steel size 6 is microscopic compared to a standard G-6 yarn hook. Check the hook's metric measurement directly or refer to the specific brand’s thread chart rather than a standard yarn-hook table.

“Size 6” yarn: This refers to the standard yarn-weight classification (Category 6: Super Bulky), not a hook measurement. The Craft Yarn Council recommends starting with a 9–15 mm hook for this weight, but always let your pattern requirements and gauge swatch guide your final pick.

What can you make with the hook you have?

Treat these ranges as a helpful starting point rather than a guarantee that every yarn in a category will pair with every listed hook. Your fiber content, stitch pattern, personal tension, and desired fabric drape will all influence the final result.

Source: Craft Yarn Council Standard Yarn Weight System. These are crochet-hook ranges, not knitting-needle ranges. Lace-weight projects are especially variable: an open shawl may use a much larger hook than a dense thread project.

Fine hooks: 2.25–3.5 mm

Use these sizes as a starting reference whenever a fingering-weight project calls for a fine hook—and consider keeping a 2.5 mm hook in your collection, as it's an especially handy size to have on hand. Keep in mind that project type often changes the rules: a designer might call for an oversized hook to achieve an airy, lacy shawl, or drop down several sizes for amigurumi to create dense, stuffing-tight stitches. Always trust your pattern’s specific instructions over the general yarn label recommendation.

Middle sizes: 3.75–6.5 mm

This range spans several yarn categories, with 4 mm and 5 mm hooks standing out as two of the most widely used "middle sizes" in crochet. Even so, don't assume hooks in this category are interchangeable—a 4 mm and a 6 mm hook will produce dramatically different results, even if both frequently appear in beginner patterns. Always check your pattern’s specific metric size, then work up a swatch with your chosen yarn. When comparing two hooks, keep the yarn and stitch identical so you can accurately isolate the hook size as the only variable.

Larger hooks: 7 mm and up

Larger tools are ideal for chunky blankets, storage baskets, and relaxed openwork, with 9 mm and 10 mm hooks being the most popular choices for super bulky yarns. How you choose depends heavily on the project: a sturdy basket requires tight, structured stitches to hold its shape, while a throw blanket needs room to drape softly. Don't be fooled by ergonomic designs—handle thickness is not your hook size. Always look for the millimeter measurement of the working shaft, no matter how chunky the grip feels in your hand.

Check your gauge before committing to the project

• Use the pattern’s yarn and stitch instructions. If you are substituting yarn, match more than the weight category: fiber and construction can change the fabric.
  
  
• Make a swatch larger than the area you will measure. For a gauge given over 4 inches, leave room around that central area so edge stitches do not distort the count.
  
  
• Wash and dry the same way the finished project will be cared for. Then measure the center and compare both stitch and row gauge with the pattern.
  
  
• Adjust one variable at a time. Too many stitches in 4 inches usually means trying a larger hook; too few usually means trying a smaller hook. Swatch again, and check that you still like the fabric.

A correct letter label does not guarantee the right gauge. Two makers can get different results with the same hook and yarn. Hook shape and the way you form stitches can also affect the result.

How to identify an unlabeled crochet hook

Inspect the entire handle, thumb rest, and shaft under good lighting for faint or faded markings. If nothing is visible, you will need to determine the diameter of the straight working shaft—never the hook head, throat, or handle grip.

A standard knitting-needle gauge ruler is rarely the right tool; the hook’s wider head often prevents the shaft from sliding through, and forcing it can warp your gauge or scratch the finish. Instead, use a specialized crochet hook gauge. These are specifically designed with open slots or dedicated cutouts to accommodate the hook head and measure the working shaft far more accurately. If you don't have one on hand, comparing your mystery tool alongside known hook sizes can help narrow down the range, but always work up a gauge swatch before starting any fitted or size-critical project.

Frequently asked questions

What is a 3.5 mm crochet hook in US size?

E-4 in the Craft Yarn Council chart. Check the millimeter marking on your own hook, especially when comparing brands or using an older tool.

What is an I-9 hook in millimeters?

The Council lists I-9 as 5.5 mm. It also lists a separate I at 5.25 mm, which is why the metric measurement is more useful than the letter alone.

Is a 4.5 mm hook a G?

The Council lists 4.5 mm as US 7, without a letter. It lists G-6 at 4 mm and G at 4.25 mm. If your manufacturer uses a different letter, follow the printed mm measurement.

What hook should I use for UK 4 ply yarn?

UK 4 ply generally refers to a fine yarn category, not US weight 4. Start with your pattern or yarn label rather than treating the number 4 as a hook size. A lace pattern and a firm sock-weight fabric can need different hooks with the same yarn.

What hook size is best for a beginner?

Start with the hook specified by a simple pattern and a smooth, light-colored yarn. A medium-weight yarn can make it easier to see individual stitches, but there is no single size that suits every beginner project.

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