US Crochet Hook Size Converter
Instantly find the exact metric (mm) equivalent of any US crochet hook letter or number size. The ultimate two-way lookup tool for crafters.
Convert Hook Sizes
Select your known US hook size or metric (mm) size below. The calculator instantly syncs to show you the exact equivalent, eliminating pattern confusion.
Crochet Hook Converter
Seamlessly switch between US letters/numbers and metric mm.
Understanding Hook Sizes
Key concepts for accurate crochet measurements
The Metric Standard
The millimetre (mm) measurement represents the actual diameter of the hook shaft. This is now the universal standard recommended by the Craft Yarn Council to eliminate regional confusion.
US Letters vs. Numbers
The US system uses letters (B, C, D…) for smaller hooks and numbers (7, 8, 9…) mid-range, before switching back to letters (L, M, N…) for very large hooks. This inconsistency is why mm is preferred.
Historical UK Sizes
Older UK patterns used a completely different number system where a higher number meant a smaller hook (similar to wire gauges). Modern UK patterns now exclusively use mm sizes.
Reading Your Hook
Most modern hooks have the mm size stamped or engraved on the flat thumb rest. If it’s worn away, use a printable hook gauge or this converter alongside a ruler to measure the shaft diameter.
Complete Crochet Hook Chart
The definitive Craft Yarn Council conversion chart for standard crochet hooks. Use this to verify your converter results or browse sizes quickly.
| Metric (mm) | US Size | Old UK/CA Size | Typical Yarn Weight |
|---|---|---|---|
| 2.25 mm | B / 1 | 13 | Super Fine (Fingering / Sock) |
| 2.75 mm | C / 2 | 12 | Fine (Sport / Baby) |
| 3.25 mm | D / 3 | 10 | Light (DK / Light Worsted) |
| 3.50 mm | E / 4 | 9 | Light (DK / Light Worsted) |
| 3.75 mm | F / 5 | 8 | Medium (Worsted / Aran) |
| 4.00 mm | G / 6 | 8 | Medium (Worsted / Aran) |
| 4.50 mm | 7 | 7 | Medium (Worsted / Aran) |
| 5.00 mm | H / 8 | 6 | Medium (Worsted / Aran) |
| 5.50 mm | I / 9 | 5 | Medium (Worsted / Aran) |
| 6.00 mm | J / 10 | 4 | Bulky (Chunky) |
| 6.50 mm | K / 10.5 | 3 | Bulky (Chunky) |
| 8.00 mm | L / 11 | 0 | Super Bulky |
| 9.00 mm | M / 13 | 00 | Super Bulky |
| 10.00 mm | N / 15 | 000 | Super Bulky / Roving |
| 11.50 mm | P / 16 | – | Super Bulky / Roving |
| 15.00 mm | Q | – | Jumbo / Roving |
| 19.00 mm | S | – | Jumbo / Roving |
| 25.00 mm | U | – | Jumbo / Roving |
Crochet Hook FAQ
Answers to the most frequently asked questions about crochet hook sizing and conversions.
You can use our converter tool above by selecting your US hook size (e.g., H/8) from the dropdown, and it will instantly display the exact metric equivalent (5.0 mm). Alternatively, you can refer to the comprehensive conversion chart provided on this page.
Historically, the US and UK used different letter and number naming conventions for crochet hooks. To eliminate global confusion, the Craft Yarn Council and international bodies now recommend using the metric millimetre (mm) measurement, which represents the actual diameter of the hook shaft, as the universal standard.
A US H/8 crochet hook is exactly 5.0 mm in diameter. This is one of the most commonly used hook sizes for medium-weight (worsted or Aran) yarns.
No. While both regions now primarily use metric (mm) sizing on modern packaging, historical US and UK letter/number systems were entirely different. For example, an old UK ‘No. 4’ hook is not the same as a US ‘D/3’ hook. Always rely on the mm measurement for accuracy.
If the engraving has worn off, you can use a printable crochet hook gauge, a digital caliper, or a standard ruler with millimetre markings to measure the diameter of the flat shaft (not including the throat or head of the hook). Then, use this converter to find the closest standard size.