Crochet Hook Sizes: Letter to mm Conversion (With Chart)

Crochet Hook Sizes at a Glance

Crochet hooks are sized three ways at once: by US letter (B through S), by US number (1 through 15 and up), and by millimeters. The millimeter measurement is the only one that's truly reliable — it describes the actual diameter of the hook's shaft, which is what determines your stitch size. A US H-8 hook is 5.0 mm; a US I-9 is 5.5 mm; the letters and numbers are just labels layered on top.

Here's the short version if you need an answer right now:

• Most-used everyday size: H-8 (5.0 mm) — the all-purpose workhorse for worsted weight yarn
• Best beginner range: H-8 to J-10 (5.0–6.0 mm) — big enough to see your stitches, small enough for standard yarn
• The rule that saves you: when a pattern and a hook disagree, trust the millimeters, not the letter

The rest of this guide gives you the full conversion chart, explains why two hooks with the same letter can be different sizes, covers the backwards world of steel hooks, and shows you how to match hook size to yarn weight.

The Full Crochet Hook Size Chart (Letter to mm)

This chart follows the Craft Yarn Council's standard sizing, which most US yarn labels and patterns use. Print it, screenshot it, tape it inside your project bag — this is the reference you'll reach for constantly.

Above 10 mm, letters get murky. Jumbo hooks are commonly labeled P, Q, or S, but the letter-to-mm mapping varies noticeably between manufacturers — a "Q" from one brand may not match a "Q" from another. At these sizes, ignore the letter entirely and buy by millimeter (15 mm, 16 mm, 19 mm, and 25 mm are common jumbo sizes).

Notice the gaps and quirks: size 7 (4.5 mm) has no letter at all, K carries the odd number 10.5, and the jump from K (6.5 mm) to L (8.0 mm) skips a full 1.5 mm. These aren't typos — the US system simply grew unevenly over decades, which is exactly why millimeters became the standard of record.

Why Two Hooks With the Same Letter Can Be Different Sizes

Here's the trap that catches crocheters who inherit hooks or buy from multiple brands: US letter sizes are not perfectly standardized between manufacturers. The best-known example is the G hook — depending on the brand, a hook stamped "G" may measure 4.0 mm or 4.25 mm. That quarter millimeter sounds trivial, but across a 150-stitch blanket row it adds up to a visibly different fabric.

Vintage hooks are even less predictable. Older hooks — especially those made before metric labeling became common — may follow sizing conventions that don't match today's chart at all.

Three habits protect you:

• Read the mm stamp, not the letter. Nearly every modern hook has the millimeter size engraved or printed on the thumb rest or shaft.
• Use a needle/hook gauge tool. It's a small plastic or metal card with sized holes; the smallest hole the shaft passes through cleanly is your true size. Essential for unmarked vintage hooks.
• Don't mix brands mid-project. Even at the "same" size, a different brand's hook shape (inline vs. tapered head) can shift your tension noticeably.

If you also knit, you'll recognize this problem — needle sizing has its own conversion headaches, which we cover in our knitting needle size chart. Happily, the millimeter solution is the same in both crafts.

Matching Hook Size to Yarn Weight

Every yarn weight has a recommended hook range, and it's printed right on the ball band next to a hook symbol. If you're not sure how to find it, our guide to reading a yarn label walks through every symbol.

Here are the Craft Yarn Council's recommended hook ranges by yarn weight:

These are starting points, not laws. A hook at the small end of the range gives you denser, stiffer fabric (good for amigurumi and bags); the large end gives you drapier, more open fabric (good for shawls and garments). For a refresher on the weight categories themselves — and why "worsted" and "aran" share a number — see yarn weights explained.

What Size Crochet Hook for Worsted Weight Yarn?

The most-asked pairing question deserves its own answer. For worsted weight (category 4) yarn:

• The standard recommendation is 5.5–6.5 mm — an I-9, J-10, or K-10.5 hook.
• In practice, many crocheters reach for an H-8 (5.0 mm) and get a slightly firmer fabric that works beautifully for dishcloths, sturdy hats, and structured bags.
• If you're a beginner buying one hook, get the H-8. It handles the widest slice of worsted patterns, and it's the size most beginner patterns assume.

Amigurumi is the notable exception: stuffed toys in worsted yarn are usually worked at 3.5–4.5 mm (E to 7) so the stuffing can't show through the stitches.

Steel Crochet Hook Sizes (Yes, They Run Backwards)

Steel hooks are the tiny hooks used for crochet thread — doilies, lace edgings, filet crochet — and their sizing works opposite to regular hooks: the bigger the number, the smaller the hook. A steel size 14 is dramatically finer than a steel size 1.

Steel hooks typically run from roughly 3.5 mm at the largest (size 00) down to well under 1 mm at the smallest — fine enough to work sewing-thread-weight cotton. Two warnings before you buy:

• Steel numbering overlaps regular numbering. A "size 7" steel hook and a "size 7" regular hook are wildly different tools. If a pattern calls for thread and a numbered hook, it means steel.
• Steel sizing varies by manufacturer even more than letter sizing does. Two brands' size 7 steel hooks may differ by several tenths of a millimeter. As always, the mm marking is the truth — check for it before you buy, and match your thread pattern's stated mm size rather than its US number when both are given.

Most crocheters working with yarn (rather than thread) will never need a steel hook. If you're just starting out, skip them until a thread project tempts you.

Crochet Hooks vs. Knitting Needles: Same Yarn, Different Numbers

If you do both crafts, don't assume the sizing systems align — they don't. A US 8 knitting needle is 5.0 mm, and so is an H-8 crochet hook, but that's coincidence, not design; the letter and number sequences diverge everywhere else. The only shared language is millimeters.

There's also a practical difference in how the sizes get used: for the same yarn, crochet patterns often call for a slightly larger mm size than knitting patterns do, because crochet stitches are denser than knit stitches. Worsted yarn that knits typically on 4.5–5.5 mm needles crochets typically on 5.5–6.5 mm hooks.

And in both crafts, the printed size is only the opening bid. Your personal tension decides the real fabric — which is why every pattern starts with a gauge swatch, and why you should actually make one for anything that needs to fit.

How to Buy Hooks Without Overthinking It

A few honest, editorial pointers to finish:

• Start with three sizes, not a 30-piece set. An H-8 (5.0 mm), an I-9 (5.5 mm), and a J-10 (6.0 mm) cover the vast majority of worsted-weight patterns. Add sizes as projects demand them.
• Aluminum hooks are the sensible default. They're smooth, durable, and inexpensive — basic aluminum hooks generally cost only a few dollars each. Ergonomic soft-grip handles cost more and are worth it if your hands ache after long sessions.
• Inline vs. tapered matters more than brand prestige. Inline heads (the hook throat cut in line with the shaft) give more uniform stitches and suit tight crocheters; tapered heads move faster and suit loose ones. Neither is "better" — try both cheaply before committing to a full set of either.
• Worth knowing: hook sets sold in gauge-skipping runs sometimes omit the in-between sizes (like 4.5 mm) that patterns actually call for. Check the mm list on the package, not the piece count.

The chart above will outlive every hook in your bag. Trust the millimeters, swatch when fit matters, and the letters will never trip you up again.