Crochet Hook Mystery Solved: H/8 Isn’t Just H
This question recently popped up in one of my beginner crochet classes and caused a bit of confusion — and it made me realize this is a mystery worth solving! A student assumed that the number attached to the U.S. letter (like the “8” in H/8) was the millimeter size. Needless to say, they arrived at class with a hook that wasn’t quite right for the project. Others then asked, “What does that other number mean?”
Honestly, that same question had crossed my mind a few times, but I’d never taken the time to look into it.
The Basics of Crochet Hook Sizing
Most of us are familiar with the general systems used to size crochet hooks. Here’s a quick rundown:
Metric Sizing: The most straightforward notation, listed in millimeters (mm). This number refers to the diameter of the hook’s shaft (the widest part).
U.S. Sizing: Uses letters. Lower letters (A, B, C) represent smaller hooks, while higher letters (I, J, K) are larger.
You may also see reference to U.K. sizing in older materials, but in the U.S., most hooks and patterns rely on either the metric (mm) or U.S. letter system.
The Mystery of the Extra Number
Sometimes you’ll see a U.S. letter followed by a seemingly random number that doesn’t match the millimeters — for example, H/8 – 5.0 mm.
So what does that extra number mean? It must have some importance — otherwise, why list it at all?
I decided to find out. I scoured every book in my crochet library, did an extensive online search, and even asked ChatGPT (my last resort!). All I found were conversion charts and explanations of the differences between U.S. and metric sizing. No one explained the additional number (H/8, I/9, J/10, etc.).
The Epiphany
Then I came across a sizing chart that included knitting needles as well — and I finally noticed something!
Those extra numbers increase by one as the sizing gets larger, exactly like knitting needle sizes. Knitting needles also have U.S. and metric measurements, but their U.S. sizing uses numbers instead of letters — the higher the number, the larger the needle.
When I compared the crochet hook chart to the knitting needle chart, everything clicked.
That extra number on the crochet hook corresponds to the U.S. knitting needle size!
Both share the same millimeter measurement.
It seems so obvious now — I felt silly for never noticing it before!
What About the “In-Between” Sizes?
In theory, if a U.S. crochet hook size includes that extra number, it lines up neatly with the even increments used for knitting needles.
If the sizing lists only a letter (with no number), that usually indicates an in-between size that doesn’t exist in knitting.
Knitting needles are typically made only in full or half-millimeter sizes (5.0, 5.5, 6.0, and so on), while crochet hooks can go down to quarter-millimeter sizes (5.0, 5.25, 5.5, etc.).
For example:
A U.S. I hook (no number) is 5.25 mm.
A U.S. J hook (no number) is 5.75 mm.
The Catch: Manufacturing Inconsistencies
Of course, nothing in the crochet world is ever completely straightforward.
While the knitting needle correspondence is the intended standard, not every manufacturer follows it — which can be incredibly frustrating! You may come across hooks labeled the same but sized slightly differently in millimeters. For instance, two hooks both marked “H” might actually measure differently.
So What’s a Crocheter to Do?
Here are my two essential pieces of advice:
Get comfortable with metric measurements.
Metric sizing is the most accurate and consistent way to measure hooks. As I tell my students, millimeters don’t lie!Always make a gauge swatch.
I know, I know — it feels boring and time-consuming (even though it actually saves time in the long run). The hook size listed in a pattern is a suggestion, not a requirement. Use whatever hook you need to match the pattern gauge. Experiment until your project comes out as intended.
The Takeaway
So the next time you see an extra number on your crochet hook, you’ll know it’s not random at all — it’s a friendly nod to our knitting cousins. And as always, trust your gauge and your millimeters!
For more information on hook & needle sizing check out the Craft Yarn Council.
