Crochet Hook Size Converter: Metric, US & UK Conversion Tool

Crochet hook sizing is one of the most confusing aspects of the craft, especially when following patterns from different countries. Three distinct sizing systems exist worldwide: the metric system (measured in millimeters), the US letter/number system common in North American patterns, and the UK/Canadian numbering system used in British and Commonwealth patterns. To make matters more complex, the UK system runs in reverse — smaller numbers indicate larger hooks, which is the opposite of what most crafters expect.

Our free hook size converter tool eliminates this confusion instantly. Enter any hook size in one system, and see the equivalent in all three. Whether you've found a beautiful vintage pattern that calls for a "UK size 9" hook, or a modern American pattern specifying an "H/8," you'll know exactly which millimeter size to reach for in your hook collection.

The Craft Yarn Council has worked to standardize hook sizing around the metric system, and most modern hooks sold worldwide now display their millimeter size prominently. However, many popular pattern books and websites still reference US or UK sizes, making a reliable conversion tool essential for any crocheter's toolkit. Below you'll find both our interactive converter and a complete static reference table.

Hook material also affects your experience, even at the same size. Aluminum hooks are smooth and fast, making them ideal for acrylic and cotton yarns. Bamboo and wooden hooks provide more grip, which helps control slippery yarns like silk and bamboo fiber. Ergonomic hooks feature cushioned handles to reduce hand fatigue during long sessions. Regardless of material, the sizing is standardized — a 5.0 mm hook is 5.0 mm whether it's aluminum, bamboo, or ergonomic. For detailed guidance, see our complete hook size chart article.

Complete Crochet Hook Size Reference Table

This table includes all standard crochet hook sizes used worldwide. Sizes marked with "—" indicate that no equivalent exists in that particular system.

History of Hook Sizing Systems

The US letter/number system was developed by American hook manufacturers in the early 20th century. Letters were assigned alphabetically as hook sizes increased, with "B" being one of the smallest common sizes and progressing through the alphabet. The number designations (like "/8" in "H/8") were added later to provide an alternative reference. This system became standard in North American patterns published by companies like Leisure Arts, Annie's, and Red Heart.

The UK/Canadian system predates both the US and metric systems and uses numbers in reverse order — a UK size 14 is one of the smallest (2.0 mm), while a UK 000 is one of the largest (10.0 mm). This system was based on the Steelmaker's Wire Gauge, which was originally used to measure wire diameter. As crochet hooks were traditionally made from steel, this gauge was adapted for hook sizing. Today, UK sizing is common in vintage patterns and patterns published in Britain, Australia, and New Zealand.

Hook Materials and How They Affect Stitching

The material your hook is made from influences your crochet experience beyond just the size number. Aluminum hooks are the most common and affordable — they're lightweight, smooth, and allow yarn to glide quickly, making them ideal for speed crocheters. However, they can cause hand fatigue during extended sessions due to their thin profile.

Bamboo and wooden hooks provide a warmer, more natural feel and offer more friction against the yarn. This extra grip is beneficial when working with slippery fibers like silk, bamboo, or mercerized cotton. They're also quieter than metal hooks, which some crocheters prefer. Ergonomic hooks feature cushioned or contoured handles (often made of rubber or silicone) designed to reduce strain on the thumb, index finger, and wrist — they use the same standardized hook sizes but with a drastically improved comfort profile.

Frequently Asked Questions