We have all hit this specific roadblock: you are ready to cast on, but the pattern speaks one language ('Size G') and your hooks speak another (millimeters). It is a universal headache. It gets even trickier if you are trying to use those beautiful vintage hooks you found or inherited, only to realize their sizing numbers seem to completely defy modern logic.
If you have ever stood in the craft aisle staring at a wall of aluminum and bamboo, wondering if 4.0mm is the same as a G or an H, you are certainly not alone. It is one of the most common hurdles we face in the fiber arts community. We speak a universal language of stitches—single, double, treble—but our tools often speak different dialects depending on where they were manufactured.
Understanding these sizes isn't just about memorizing a chart; it is about gaining control over your fabric. The hook size dictates the drape of a shawl, the stiffness of a basket, and the safety of an amigurumi toy. When you master the conversion, you stop following patterns blindly and start engineering your projects to look exactly how you envisioned them.
Understanding Crochet Hook Sizes
To the uninitiated, a hook is a hook. But for us, the difference of half a millimeter is the difference between a beanie that fits perfectly and one that slides right over your eyes. The confusion stems from the fact that we have three dominant labeling systems operating simultaneously in the market: Metric, US, and the older UK/Canadian system.
The Metric System (Millimeters)
This is the gold standard. It is precise, logical, and universal. When you see a hook labeled "5.0mm," it refers to the diameter of the hook's shaft—the straight part between the thumb grip and the tapered throat. This is where the yarn loops sit while you work, determining the size of your stitch. Regardless of brand or country of origin, 5.0mm is physically 5.0mm. I always encourage my readers to look at the millimeter size first, even if they are used to letters, because it cuts through the marketing variations.
The US System (Letters and Numbers)
If you are buying hooks in North America, you are likely dealing with an alphanumeric soup. We use letters (B, C, D...) and sometimes numbers to denote size. As you move further down the alphabet, the hook gets thicker. A "B" hook is tiny; a "Q" hook is massive. The trouble arises because this system is arbitrary. There is no mathematical reason why "H" equals 5.0mm; it was simply assigned that way. Furthermore, different manufacturers sometimes disagree on what millimeter corresponds to which letter, particularly in the middle range of sizes.
The UK and Canadian System (Numbers)
This system often confuses beginners because it works inversely to the US system. In the old UK sizing, the larger the number, the smaller the hook. A size 14 hook is very fine, suitable for thread, while a size 000 is a large, chunky hook. Most modern UK patterns have switched to metric to avoid confusion, but if you are working from a vintage pattern found at an estate sale, you need to be aware of this inversion.
Patricia's Pro-Tip: I've seen many clients make the mistake of buying a "discount set" of hooks online that don't have millimeter markings. The simple fix is to invest in a cheap plastic knitting gauge tool. These little rulers have holes drilled in them. You can slide your mystery hook into the holes to find its true metric size, regardless of what letter is stamped on the handle.
The Importance of the Shaft
When we talk about size, we are strictly talking about the shaft. The head style—whether it is "inline" (Bates style) or "tapered" (Boye style)—does not affect the official size, though it drastically changes the fishing experience. However, the shaft is the law. If you crochet tightly, you might be forming your stitches on the throat (the tapered part) rather than the shaft. This effectively means you are using a smaller hook than you think.
Standardization Struggles
The Craft Yarn Council has done a tremendous amount of work to standardize these conversions, but variances still exist. I have two "G" hooks in my studio right now. One is 4.0mm, and the other, an older oversized brand, is closer to 4.25mm. That tiny fraction seems negligible, but over the course of a blanket with 20,000 stitches, it adds up to a significant difference in width.
US Crochet Hook Sizes Chart
Navigating the conversion between US letters and Metric millimeters is a rite of passage. While memorizing every single conversion isn't necessary, having a reliable reference is vital. Below is the breakdown of the standard alignment accepted by most major yarn companies and pattern designers today.
| US Size | Metric (mm) | Common Yarn Weight Pairing |
| B-1 | 2.25 mm | Super Fine (1) / Sock |
| C-2 | 2.75 mm | Fine (2) / Sport |
| D-3 | 3.25 mm | Fine (2) / Sport |
| E-4 | 3.50 mm | Light (3) / DK |
| F-5 | 3.75 mm | Light (3) / DK |
| G-6 | 4.00 mm | Medium (4) / Worsted |
| 7 | 4.50 mm | Medium (4) / Worsted |
| H-8 | 5.00 mm | Medium (4) / Worsted |
| I-9 | 5.50 mm | Medium (4) / Worsted / Aran |
| J-10 | 6.00 mm | Bulky (5) |
| K-10.5 | 6.50 mm | Bulky (5) |
| L-11 | 8.00 mm | Super Bulky (6) |
| M/N-13 | 9.00 mm | Super Bulky (6) |
| N/P-15 | 10.00 mm | Super Bulky (6) |
The Steel Hook Exception
It is important to note that "Thread" hooks use a completely different numbering system in the US. These are the tiny steel hooks used for doilies and fine lace. They use numbers like 10, 12, or 8. In this specific category, similar to the old UK system, the higher the number, the smaller the hook. A size 14 steel hook is microscopic compared to a size 0.
The "7" Hook Anomaly
You will notice the size "7" sitting between G and H. This is 4.5mm. It is an incredibly versatile size that often gets overlooked because it doesn't have a catchy letter associated with it in most sets. Some brands call it a G+, while others just label it "7". If you find your G hook is too tight but your H hook leaves gaps, the 4.5mm is your savior.
Variances in "K" and Up
Once you get into the larger sizes (K, L, M, N), standardization gets a little loose. One brand's N hook might be 9.0mm while another is 10.0mm. Always double-check the millimeter sizing printed on the packaging of chunky yarns and hooks to ensure they match up.
Common Conversions: What is an H Hook in MM?
If there is a celebrity in the world of crochet hooks, it is undoubtedly the H hook. If you look through the archives of free patterns online, you will find that a massive percentage of them call for this specific size.
The H-8 is 5.0mm
To answer the question directly: A US size H-8 hook is equivalent to 5.0mm.
Why is the H Hook So Popular?
The popularity of the H hook is directly tied to the popularity of "Worsted Weight" (Category 4) yarn. For decades, Red Heart Super Saver and similar acrylic worsted yarns have been the default choice for American crafters. The 5.0mm hook is the mathematical sweet spot for this thickness of yarn. It creates a fabric that is neither too stiff nor too holey. It is perfect for afghans, scarves, and basic beanies.
The "Amigurumi" Consideration
While an H hook is standard for worsted weight clothing or blankets, it is often too large for worsted weight amigurumi. When making stuffed animals, you need a tight fabric to keep the stuffing from showing through. Beginners often grab the H hook because the yarn label suggests it, only to be disappointed when their doll looks "gappy." For those projects, you would usually drop down to a smaller size.
Visual Identification
Even without a label, you can often identify an H hook by feel if you have been crocheting long enough. It has a comfortable heft—not as spindly as the sock-weight hooks, but not as cumbersome as the bulky plastic ones. It sits nicely in the hand, which contributes to why so many beginners start their journey here.
Patricia's Pro-Tip: If you lose the label on your favorite hook and the print has rubbed off the handle (a sign of a well-loved tool!), take a piece of worsted weight yarn and hook it. Pull it gently into the throat. If the yarn sits flush with the hook head without overflowing or looking lost in space, it's likely your trusty 5.0mm H hook.
What Size is a G Crochet Hook in MM?
The G hook is the younger sibling to the H hook, often used interchangeably by relaxed crocheters, but the difference is significant enough to ruin the fit of a garment.
The G-6 is 4.0mm
Generally speaking, a US size G-6 hook converts to 4.0mm.
The 4.25mm Confusion
This is where things get tricky. Some older hook sets, particularly from brands like Boye, have historically labeled their G hooks as 4.25mm. In the metric world, 0.25mm is a legitimate step up. However, in the simplified US letter system, both 4.0mm and 4.25mm often get tossed into the "G" bucket. If you are working on a project requiring precise gauge, like a fitted sweater, you must check if your G is a 4.0 or a 4.25.
The Amigurumi Standard
For amigurumi artists using worsted weight yarn, the G hook (4.0mm) is widely considered the standard. While the yarn label might suggest an H (5.0mm), the G hook tightens the stitches just enough to create that solid, professional-looking shell that holds shape and hides stuffing. If you have been using an H hook for your toys and aren't happy with the results, switching to a G is usually the instant fix.
Handling the G Hook
Because the G hook is thinner, it can be slightly more fatiguing to hold for long periods if you have large hands or grip tightly. Many crocheters find that adding an ergonomic handle or a rubber grip is more necessary with a G hook than with an H or I hook. The shaft is thin enough that it can dig into your palm if you use the "knife hold."
Yarn Pairing
Aside from tight worsted work, the G hook is the king of DK (Double Knitting) or Light Worsted (Category 3) yarn. This is the weight often used for baby clothes and lighter shawls. A G hook with DK yarn produces a fabric with beautiful drape—fluid and soft, rather than rigid.
How to Choose the Right Hook for Your Yarn
Knowing the numbers is half the battle; knowing which one to pick up for your specific project is the art form. The suggested hook size on a yarn label is a guideline, not a law. It is a starting point recommended by the manufacturer to achieve a standard gauge. You are allowed—and often encouraged—to deviate from it.
Check the Band, Then Check Your Hand
Step 1: Read the Label. almost every skein of yarn sold today has a small square icon on the back of the paper band. Inside that square, you will see a drawing of a crochet hook with a metric number (e.g., 5mm) and a US letter (e.g., H-8). This tells you what size the manufacturer used to test the yarn.
Step 2: Analyze Your Tension. Are you a tight crocheter or a loose one? I am a notoriously tight crocheter. If a pattern calls for an H hook, I almost always have to use an I (5.5mm) hook to get the correct size. If I use the recommended H, my project comes out tiny and stiff. Conversely, if you crochet loosely, you might need to go down a size (from H to G) to prevent your sweater from becoming a dress.
Step 3: Consider the Project Type.
For Drape and Flow: If you are making a summer shawl, a lacy cover-up, or a scarf that needs to wrap elegantly, go up a hook size. Using a larger hook (like a J or K with worsted yarn) introduces more air into the stitches, making the fabric flexible.
For Structure and stiffness: If you are making a basket, a rug, or a stuffed animal, go down a hook size. You want the stitches to be dense and rigid so the object holds its own structural integrity.
The Material Factor
Believe it or not, the material of the hook affects how you choose your size.
Aluminum/Metal Hooks: These are slippery. Yarn glides over them fast. They are great for wool and acrylic that might stick to other surfaces.
Wood/Bamboo Hooks: These have "grip." They hold onto the yarn slightly. If you are working with slippery silk or bamboo yarn, a wooden hook helps you control the tension better. Sometimes, switching from a metal 5mm to a wooden 5mm can change your gauge simply because your rhythm changes.
The Golden Rule: The Gauge Swatch
I know, I know—nobody likes making gauge swatches. It feels like wasted time. But if you are unsure which hook to use, take ten minutes to crochet a 4x4 inch square. It is the only way to know for sure how that specific yarn behaves with that specific hook in your specific hands.
Frequently Asked Questions About Crochet Hook Sizes
What do I do if my hook doesn't have a letter or size on it?
This happens frequently with vintage or ergonomic hooks where the paint has worn off. The best solution is to buy a "knitting gauge" or "hook gauge" tool. It’s a small ruler with holes of various millimeter sizes. You pass the hook through the holes until you find the one that fits perfectly—snug, but not forced.
Can I use a 5.5mm hook if the pattern calls for a 5.0mm?
Yes, but with caveats. If you are making a scarf or blanket, the difference will just make your project slightly larger and softer. If you are making a fitted garment like a sweater, that 0.5mm difference could make the sweater two sizes too big. If you are making amigurumi, it might leave holes for stuffing to escape.
Why does my G hook feel smaller than my friend's G hook?
This is likely a brand discrepancy. As mentioned, some brands manufacture G hooks at 4.0mm, while others use 4.25mm. Additionally, the shape of the hook head (inline vs. tapered) changes how the yarn is grabbed, which can psychologically make the hook feel "smaller" or "fiddlier" even if the shaft is the same width.
Is a 7 hook the same as a G hook?
No. In the US system, a "7" is usually a standalone size that corresponds to 4.5mm. A G hook is 4.0mm or 4.25mm. The 7 sits right between the G and the H (5.0mm). It is a fantastic bridge size if G is too tight and H is too loose.
Do different yarn colors affect hook choice?
Surprisingly, yes! darker dyed yarns (like black or navy) can sometimes be slightly thinner or stiffer due to the heavy saturation of dye chemicals. You might find that you need to adjust your tension or even your hook size slightly when switching from a white yarn to a black yarn of the exact same brand.
Crochet is an art, but it is built on a foundation of engineering. Understanding your tools—specifically the critical difference between US letters and metric millimeters—gives you the power to adapt any pattern to your style. Don't be afraid to experiment with different sizes to see how they change the fabric of your work. The next time you pick up a skein of yarn, look past the recommended size on the label and ask yourself: "What kind of fabric do I want to create today?" Then, choose your hook accordingly.
