Crochet Hook Sizes: The Complete Chart and Conversions

Crochet hook sizes can look confusing at first, with letters, numbers, and millimeters all printed on the packaging, but the system is simpler than it seems. A hook's size is just the thickness of its shaft, and that thickness decides how big your stitches are and which yarn it suits. Once you understand how the sizing works and can read the numbers at a glance, buying hooks and following patterns becomes easy. This guide covers the metric and US systems, steel thread hooks, and how to convert between them. For the wider picture of hooks, start with the main crochet hooks guide.

How Hook Sizes Are Measured

A crochet hook's size is the diameter of its shaft, which is the straight section just below the head where each stitch is formed. This is measured in millimeters, so a 5.0 mm hook has a shaft that is five millimeters across. Because stitches wrap around the shaft, a thicker shaft makes bigger stitches and a thinner shaft makes smaller ones. That is the whole idea behind hook sizing: the number tells you how big your stitches will be, which in turn tells you what yarn and project the hook suits.

Metric Is the Universal Standard

Millimeters are the standard used around the world, and they are the most reliable way to talk about hook size because they measure the actual diameter. Whatever country a pattern comes from, a 4.0 mm hook is a 4.0 mm hook. For this reason, it is worth getting used to thinking in millimeters, and when you buy hooks, the millimeter size is the number to trust. If a pattern lists only a letter size, look up its millimeter equivalent to be sure you have the right one, since letters are less consistent.

US Letter and Number Sizes

In the United States, hooks also carry a letter and number label, such as B-1, G-6, H-8, or K-10.5. These correspond to metric sizes, so H-8 is 5.0 mm and G-6 is 4.0 mm. The letters generally increase with size, so a larger letter means a bigger hook. The catch is that these labels can vary slightly between brands, and the same letter might be a hair different in millimeters from one maker to another. That is why experienced crocheters treat the letter as a helpful label but rely on the millimeter number underneath it.

Common Sizes and What They Suit

A handful of sizes cover most crochet. Small hooks around 2.25 mm to 3.5 mm suit fine yarns like lace and fingering. Medium hooks around 4.0 mm to 5.5 mm suit DK and worsted yarns and are where most everyday crochet happens, with the 5.0 mm hook being the classic all rounder. Larger hooks around 6.0 mm to 9 mm suit bulky yarns and quick, cozy projects. Jumbo hooks of 10 mm and up handle super bulky and jumbo yarn. Matching hook size to yarn is a skill in itself, covered in which hook for your yarn.

Steel Hooks for Thread

There is a separate family of very small hooks called steel hooks, used for fine thread crochet, lace, and delicate doilies. These use their own numbering system, and here is the tricky part: for steel hooks, a higher number means a smaller hook, which is the reverse of the regular system. A size 7 steel hook is smaller than a size 1. Because of this reversal, it is easy to get confused, so always check the millimeter size printed alongside the steel number to be sure you have the size you actually want.

US and UK Terms Can Differ

If you use patterns from different countries, be aware that older or region specific charts sometimes list hook sizes differently, and the UK historically used its own numbering too. This is another reason the millimeter measurement is your safest anchor, because it does not change between regions. When a vintage or international pattern gives an unfamiliar size, convert it to millimeters and you will always know exactly which hook to pick up. Keeping a small conversion chart handy makes working from any pattern painless.

Why the Exact Size Matters

Using the right hook size is not just about comfort, it directly affects whether your project comes out the right size. The hook controls your gauge, which is how many stitches and rows fit in a given measurement. Go up a size and your stitches get bigger, so the project grows larger and looser. Go down a size and it becomes smaller and denser. For anything that needs to fit, like a hat or a garment, matching the hook to hit the pattern's gauge is essential, a habit explained in the crochet basics.

Choosing the Right Size

Understanding hook sizes gives you the confidence to pick the right tool for any yarn and pattern. Remember that the millimeter number is the value to trust, that 5.0 mm is the friendly all rounder, and that steel hooks run backward. From there, matching a size to your yarn and adjusting to hit gauge becomes second nature. To go further, see which hook for your yarn and how to choose a crochet hook, or head back to the crochet hooks guide for the full overview.