Crochet Hook Size Reference

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Walk into any yarn shop and you will find walls of hooks in every imaginable size, labelled with a bewildering mix of millimetre measurements, US letter sizes, and UK old-standard numbers. A pattern calls for a 5 mm hook - is that a US H/8 or a US I/9? And what is the UK equivalent? The Crochet Hook Size Reference answers all of these questions instantly, serving as your go-to conversion chart for every crochet hook sizing system in use today.

Why Hook Size Matters So Much

Using the wrong hook size is the number-one reason crochet projects come out the wrong size. Every crochet pattern is designed around a specific hook size and a specific yarn weight. Together, they determine your gauge - the number of stitches and rows per inch. Change the hook size by even half a millimetre and your gauge shifts, which means your finished blanket, garment, or amigurumi could end up noticeably larger or smaller than intended.

This is especially critical for wearable projects. A sweater crocheted at the wrong gauge might be unwearable. A hat could turn out too tight or too loose. Even for non-wearable projects like blankets or bags, getting the right hook size ensures proper drape, stitch definition, and yarn consumption - meaning you are less likely to run out of yarn three rows from the end.

The Three Major Sizing Systems

Metric (millimetres). The most universal and precise system. Hook sizes are expressed as their actual shaft diameter: 2.0 mm, 2.25 mm, 2.5 mm, and so on. Most modern patterns worldwide use metric sizing, and it is the system this reference guide treats as the anchor for all conversions.

US sizes. American patterns use a letter-number combination: B/1, C/2, D/3, up through S/35 for the largest hooks. The problem is that the letter progression is not perfectly consistent - there is no standard for some sizes, and different manufacturers have historically assigned slightly different millimetre values to the same letter.

UK/Canadian old standard. Older British and Canadian patterns use a numbered system that runs in the opposite direction from US numbers - a UK 14 is a tiny 2.0 mm hook, while a UK 2 is a large 7.0 mm. If you are working from a vintage pattern, this can cause serious confusion without a reliable reference.

What This Reference Includes

The Crochet Hook Size Reference provides a comprehensive cross-reference table covering:

All standard sizes from 0.6 mm (steel hooks for thread crochet) through 25 mm (jumbo hooks for arm-knitting-weight yarn), with US and UK equivalents for each metric size.

Steel hook sizes for lace and doily work, which use their own separate numbering system in the US (steel sizes 00 through 14) that does not overlap with regular hook numbering.

Recommended yarn weights for each hook size range - lace weight for 1.5-2.25 mm hooks, fingering for 2.25-3.5 mm, DK for 3.5-4.5 mm, worsted for 5.0-5.5 mm, bulky for 6.0-8.0 mm, and super bulky for 9.0 mm and up.

When to Size Up or Down

The reference also includes practical advice on when to intentionally use a different hook size than the pattern recommends. Tight crocheters - people who naturally pull their yarn snug - often need to go up one hook size to match gauge. Loose crocheters may need to go down. For amigurumi (stuffed toys), most makers deliberately use a hook one or two sizes smaller than the yarn label suggests to create a tight, firm fabric that does not show gaps when stuffed.

Always at Your Fingertips

The Crochet Hook Size Reference runs in your browser with no downloads and no account. Bookmark it on your phone and you will never again stand in a yarn shop squinting at hook packages trying to figure out whether a US G/6 is the same as 4.0 mm or 4.25 mm. Quick, accurate, and free - exactly what every crocheter needs.