Crochet Hook Size Converter

Crochet hook sizes use three different numbering systems depending on the country: US letter/number, metric millimeters, and old UK/Canadian numbers. The metric measurement is the actual diameter of the hook shaft, which determines stitch size and fabric density.

How the three systems compare:

US System: Uses a combination of letters (B through S) and numbers. The system goes from B/1 (2.25 mm, smallest common) up to S (19 mm, jumbo). Letters progress alphabetically with increasing size. Some sizes use a number alongside the letter (e.g., G/6, H/8). This system is standard in patterns published in the United States.

Metric System (mm): Measures the actual hook shaft diameter in millimeters. This is the most precise and universal system because it leaves no ambiguity. Metric sizes range from about 0.75 mm (steel hooks for fine lace, and a little finer on some vintage charts) up to 25 mm or more for jumbo hooks. The metric system is used worldwide and is the recommended system for international patterns.

Old UK/Canadian System: Similar to old UK knitting needle sizes, the numbering runs in reverse, so higher numbers mean smaller hooks. A UK 14 is tiny (2.0 mm) while a UK 2 is large (7.0 mm). This system is mostly obsolete but still appears in vintage patterns and some British publications. Modern UK patterns now typically use metric sizes.

Steel crochet hooks (for thread crochet): Steel hooks are used for fine crochet thread and lace work. They use a separate numbering system that runs the opposite way to the letter hooks: 00 is the largest at about 3.5 mm, and the numbers climb as the hook shrinks, down to a 14 at roughly 0.75 mm. Brands disagree at the fine end by up to about 0.2 mm, and a few vintage charts take size 14 as low as 0.6 mm, so trust the millimetre stamped on the hook over the number.

Steel hooks are also shorter than standard aluminium or plastic hooks and have a flattened thumb rest instead of a full handle.

Full cross-reference table:

A dash means there is no hook in that system at that diameter. The old UK range stops at 2 (7.0 mm); anything larger was sold by the millimetre, which is why the column empties out at the bottom.

Choosing the right hook size: The yarn label always recommends a hook size range. However, individual crochet tension varies significantly from person to person. The best practice is to make a gauge swatch: crochet a 4-inch (10 cm) square using the recommended hook and count the stitches. If you have too many stitches, switch to a larger hook. If too few, go smaller.

Hook materials and their characteristics:

• Aluminum: Smooth, fast, lightweight. The most popular material for standard hooks.
• Steel: Required for fine thread work. Very smooth and strong despite the small diameter.
• Bamboo/Wood: Warm to the touch, slightly grippy surface helps prevent dropped stitches. Good for slippery yarns.
• Plastic: Lightweight and affordable. Available in very large sizes for super-bulky yarn.
• Ergonomic handles: Many modern hooks feature cushioned or contoured handles to reduce hand fatigue during long crochet sessions.

Inline vs tapered hooks: Inline hooks (like Susan Bates brand) have a hook that is the same diameter as the shaft, with a deeper throat. Tapered hooks (like Boye brand) have a more rounded head with a shallower throat. Neither is better. It is a matter of personal preference and crocheting style.