Why Crochet Hook Sizes Are Such a Mess (And How to Actually Deal With It)

If you’ve ever bought a “size G” crochet hook from two different brands and found they were noticeably different thicknesses, you weren’t imagining it. You also weren’t doing anything wrong. Crochet hook sizing is genuinely inconsistent, and the reason has more to do with 20th-century manufacturing history than with anything you did at the checkout.

There Is No Single Standard

This is the part most charts don’t say out loud: there is no single international body that defines what “size G” means in millimeters, the way there is for, say, screw threads. The closest thing is the Craft Yarn Council, a US industry group that publishes a recommended letter-to-millimeter table. Manufacturers are free to follow it, adjust it slightly, or ignore it, and plenty do all three.

That’s why our own crochet hook size chart leads with millimeters rather than letters — millimeters are a physical measurement, and a physical measurement can’t disagree with itself. A letter is a label a company chose to put on a package, and different companies have chosen differently.

How the Letter System Actually Got Started

The US letter-and-number system (B, C, D… through the low numbers, then letters again at the large end) has roots in 19th- and early-20th-century hook manufacturing, when hook sizes were tied to the gauge of steel wire a company happened to draw. Different mills drew wire to slightly different gauges, so “a size 7 hook” from one maker and “a size 7 hook” from another could vary by a few hundredths of a millimeter — usually not enough to matter for hand crochet, but enough that when the industry later tried to standardize, nobody could agree on a single clean mapping from letter to millimeter without someone’s existing tooling becoming “wrong.”

The Craft Yarn Council’s table, which most US charts (including ours) use today, is really a best-effort peace treaty between decades of slightly different manufacturer conventions. It’s a good one. It’s just not a law of physics.

Steel Hooks Use an Entirely Different Numbering System

Here’s the trap that catches a lot of people who are used to regular yarn hooks: steel thread hooks — the fine ones used for lace and thread crochet — use a separate numbering system that runs backward. A steel size 7 is roughly 1.65 mm. A steel size 12 is smaller still, around 0.75 mm. Higher number, smaller hook — the opposite direction from the aluminum/plastic yarn-hook range, where a bigger number or later letter usually means a bigger hook.

If you’ve ever wondered why a “size 7” steel hook from your grandmother’s sewing box looks nothing like a “size 7 (4.5 mm)” hook from the craft store, this is why. They’re not the same numbering system wearing the same digit by coincidence — they’re two systems that happen to share a symbol.

UK Sizing Runs Backward Too, and Is Basically Retired

Older UK crochet patterns use a numbered system where, confusingly, a higher number means a smaller hook — a UK 2.00 is roughly a US H-8, while a UK 14 is a fine steel-range size. This system was in wide use in Britain before metric sizing took over, and it still turns up in vintage pattern books and inherited hook sets.

Most contemporary British patterns have moved to millimeters directly, which sidesteps the whole problem — which is also why our chart doesn’t try to publish a UK column at all. A UK-numbered vintage hook is worth measuring directly rather than trusting a remembered conversion, because the old UK system was applied inconsistently even in its own time.

Buying a Multi-Pack of “Assorted” Hooks? Check the Millimeters, Not the Letters

If you’re shopping for a hook set — especially an inexpensive multi-pack — look at the millimeter marking stamped on the hook itself, not just the letter printed on the packaging. It’s common for budget sets to round to the nearest “friendly” letter even when the actual shaft is a fraction of a millimeter off the Craft Yarn Council recommendation. For most projects that’s genuinely fine. For anything where gauge is tight — garments, amigurumi that needs to match a pattern’s exact stitch count — it’s worth measuring the hook yourself once, rather than trusting the label twice.

That’s the whole reason a screen-calibrated gauge is worth more than a printed chart for an unmarked or uncertain hook: a chart can only tell you what a size is supposed to measure. It can’t measure the hook actually in your hand.

Does the Hook’s Material Change Its Size?

Not the printed number, but it can change how a hook of that number behaves, which is a separate thing worth untangling. An aluminum 5 mm hook and a bamboo 5 mm hook both measure 5 mm across the shaft — the size label refers strictly to that diameter, regardless of what the hook is made from.

What material changes is friction and, indirectly, the tension a lot of crocheters end up working at. Metal hooks are slick, which lets yarn slide with less resistance and tends to produce looser, more relaxed stitches for a given amount of hand tension. Wood and bamboo grip the yarn more, which tends to produce tighter stitches at the same tension, because the crocheter’s hands are unconsciously compensating for the extra drag. Neither effect shows up on a chart or a caliper measurement — the shaft is genuinely 5 mm either way — but it’s a real reason two people, or even one person switching hook materials mid-project, can get visibly different gauge from what the label promises. If a project’s finished size matters, swatching with the actual hook you’ll use — not just the size you’ll use — is worth the ten minutes.

Inherited and Secondhand Hook Sets

Vintage and inherited hook sets are common — crochet supplies get passed down and hooks last essentially forever, so a set from decades ago is often still perfectly usable. The catch is that older sets are exactly where the UK numbering system, older manufacturer conventions, and unmarked steel hooks all tend to turn up together, often in the same tin.

Before assuming a vintage hook matches a modern chart, it’s worth measuring the shaft directly rather than trusting a printed number that might follow a retired convention. If a hook has no visible marking at all — common on older steel hooks, where the stamp can wear away with decades of handling — a screen-calibrated gauge is the fastest way to identify it without needing a caliper, since it only requires holding the hook up to a device you already have.

The Practical Takeaway

You don’t need to memorize any of this history to crochet well. The short version that actually matters at the yarn store:

• Trust the millimeter marking over the letter, always — it’s the only figure that means the same thing everywhere.
• Steel hooks are a different system. A steel 7 and an aluminum 7 have nothing in common except the digit.
• Vintage and UK-numbered hooks should be measured, not converted from memory — the old systems were inconsistent even among themselves.
• When a pattern’s gauge really matters, swatch with the actual hook you’re using, not the hook the label says you’re using.

Every hook is, in the end, just a shaft of a certain diameter. The letters and numbers are conveniences people invented on top of that fact, at different times, in different countries, for different reasons — which is exactly why they don’t always agree with each other.