Crochet Hook & Yarn Converter
Hook sizes across metric, US, and UK systems, a gauge calculator for when your swatch doesn't match the pattern, and the standard yarn weight table — all in one place.
Hook Size Converter
Metric millimetres are the one unambiguous measurement — US letters and UK numbers both reuse digits for different sizes.
5 mm = US H-8 · UK 6
| Metric | US | UK |
|---|---|---|
| 2 mm | — | 14 |
| 2.25 mm | B-1 | 13 |
| 2.5 mm | — | 12 |
| 2.75 mm | C-2 | — |
| 3 mm | — | 11 |
| 3.25 mm | D-3 | 10 |
| 3.5 mm | E-4 | 9 |
| 3.75 mm | F-5 | — |
| 4 mm | G-6 | 8 |
| 4.5 mm | 7 | 7 |
| 5 mm | H-8 | 6 |
| 5.5 mm | I-9 | 5 |
| 6 mm | J-10 | 4 |
| 6.5 mm | K-10.5 | 3 |
| 7 mm | — | 2 |
| 8 mm | L-11 | 0 |
| 9 mm | M/N-13 | 00 |
| 10 mm | N/P-15 | 000 |
| 12 mm | — | — |
| 15 mm | P/Q | — |
| 16 mm | Q | — |
| 19 mm | S | — |
Gauge Calculator
All gauges are stitches across a 4 in / 10 cm swatch. Work a swatch, count honestly, and this tells you how far off the pattern you'll land.
At your gauge, the pattern comes out 6.7% smaller.
For 40 in at your gauge: start with about 150 stitches.
Yarn Weight Reference
The Craft Yarn Council's standard system. Wraps per inch (WPI) is the one you can measure from an unlabelled ball.
| # | Name | Ply | Hook | WPI | sc / 4 in |
|---|---|---|---|---|---|
| 0 | Lace / Thread | 1-3 ply | 1.5-2.25 mm | 30-40+ | 32-42 sts |
| 1 | Super Fine / Fingering | 4 ply | 2.25-3.5 mm | 14-30 | 21-32 sts |
| 2 | Fine / Sport | 5 ply | 3.5-4.5 mm | 12-18 | 16-20 sts |
| 3 | Light / DK | 8 ply | 4.5-5.5 mm | 11-15 | 12-17 sts |
| 4 | Medium / Worsted / Aran | 10 ply | 5.5-6.5 mm | 9-12 | 11-14 sts |
| 5 | Bulky / Chunky | 12 ply | 6.5-9 mm | 7-9 | 8-11 sts |
| 6 | Super Bulky | — | 9-15 mm | 5-7 | 6-9 sts |
| 7 | Jumbo | — | 15+ mm | 0-4 | 0-6 sts |
Why Gauge Matters More Than Hook Size
The hook size in a pattern is a starting suggestion, not an instruction. What actually determines whether a sweater fits or a blanket hits its dimensions is gauge — the number of stitches and rows you get over a measured swatch. Two people using the same hook and yarn can be a full hook size apart in gauge because of how tightly they hold the working yarn. Swatch first, measure honestly, and adjust your hook until the numbers match. On a small project it feels like a waste of twenty minutes; on a blanket it's the difference between the size you wanted and a foot of extra fabric.
Frequently Asked Questions
What is a 5.0 mm hook in US and UK sizes?
A 5.0 mm hook is a US H-8 and a UK 6. Because US letter sizes and UK numbers were assigned separately, the same number can mean different hooks — the millimetre measurement is the only one that's unambiguous, so patterns increasingly list it first.
Why does my finished piece come out the wrong size when I followed the pattern?
Almost always gauge. If you get more stitches per 4 inches than the pattern, your piece comes out smaller; fewer, and it comes out bigger. Work a swatch in the pattern stitch, measure it, and change your hook size — not your tension — until your gauge matches.
How do I identify yarn weight without a ball band?
Wrap the yarn snugly (not stretched) around a ruler for one inch and count the wraps. Roughly: 30-40+ is lace, 14-30 fingering, 12-18 sport, 11-15 DK, 9-12 worsted, 7-9 bulky, and under 7 super bulky.
Does hook size change how much yarn I need?
Yes. A larger hook makes looser, larger stitches, so you cover more area per yard — but the fabric is more open. A smaller hook uses more yarn for the same dimensions and gives a denser fabric. Substituting a hook size always changes yardage.