Yarn & tools
Crochet Hook Sizes Explained: Millimeters, US Letters and Matching Yarn
Crochet hooks are sized by the diameter of the shaft in millimeters. US hooks add a letter and number, so 5 mm is H-8 and 6 mm is J-10. Thicker yarn needs a bigger hook: medium (worsted) yarn usually pairs with 5.5 to 6.5 mm, bulky with 6.5 to 9 mm. When in doubt, go by the mm size.
By the Knit&Note team
A crochet hook size is a measurement of the hook's shaft, the part your loops sit on, given in millimeters. In the US, hooks also carry a letter and a number, which is handy until two brands disagree.
This guide gives you the standard conversions, the yarn weight that usually goes with each hook, and when it makes sense to break the rules.
How hook sizes are measured
The millimeter size is the diameter of the shaft, the straight section between the hook's throat and the grip. That diameter decides how big each loop is, and so how big your stitches are.
The US system adds a letter and a number, so a US pattern might ask for a 'size H-8 (5 mm) hook'. Both go up as the hook gets bigger. When a pattern gives both, go by the millimeters. Letters can vary slightly between brands, but 5 mm is always 5 mm.
Older UK patterns sometimes use a numbered system where a bigger number meant a smaller hook. Modern UK patterns list millimeters, so you will mostly meet this in vintage patterns.
Crochet hook size chart: mm to US
2.25 mm = B-1
2.75 mm = C-2
3.25 mm = D-3
3.5 mm = E-4
3.75 mm = F-5
4 mm = G-6
4.5 mm = 7 (often sold without a letter)
5 mm = H-8
5.5 mm = I-9
6 mm = J-10
6.5 mm = K-10.5
8 mm = L-11
9 mm = M/N-13
10 mm = N/P-15
Some brands make in-between sizes, such as 7 mm, that do not always have a US letter. Steel hooks for thread crochet use their own numbering, where a higher number means a smaller hook. For both, match the millimeters, not the label.
Which hook goes with which yarn weight
Every yarn label lists a recommended hook size, and that is the best starting point. As a general guide, these are the crochet hook ranges the Craft Yarn Council suggests for each standard yarn weight:
Lace (0): steel hooks, or a regular 2.25 mm (B-1)
Super fine (1), sock and fingering: 2.25 to 3.5 mm (B-1 to E-4)
Fine (2), sport: 3.5 to 4.5 mm (E-4 to 7)
Light (3), DK and light worsted: 4.5 to 5.5 mm (7 to I-9)
Medium (4), worsted and aran: 5.5 to 6.5 mm (I-9 to K-10.5)
Bulky (5), chunky: 6.5 to 9 mm (K-10.5 to M/N-13)
Super bulky (6): 9 to 15 mm (M/N-13 to Q)
Jumbo (7): 15 mm and larger (Q and up)
These are starting points, not laws. The pattern always wins over the chart, because the designer chose the hook for the fabric they wanted.
When to go up or down a hook size
To match gauge. If your swatch has more stitches than the pattern, go up a size. Fewer stitches, go down. This is the main reason to change hooks, and it is personal: two people with the same yarn and hook can get different fabric.
For dense, sturdy fabric. Bags, baskets and amigurumi often use a smaller hook than the yarn label suggests, so the stitches close up, the fabric holds its shape and the stuffing stays inside.
For drape. Shawls, wraps and some garments use a larger hook for a softer, more flowing fabric.
For the foundation chain. If your starting chain always pulls tighter than the rows above it, make the chain with a hook one size up, then switch back for the first row.
For a garment like June Jacket, check gauge before you commit. For a bag, getting close is usually enough, as long as the fabric feels firm.
Choosing your first hooks
If you are building a collection, a few sizes cover most beginner projects: 4 mm, 5 mm, 6 mm and 8 mm. Add others when a pattern asks for them.
Pay attention to the shape as well as the size. On an inline hook, the head is the same width as the shaft and the throat is cut straight in. On a tapered hook, the throat narrows and the head is rounder. They feel different in the hand, and most crocheters end up with a favorite. A soft, ergonomic grip is kind to your hand on long projects.
In the Knit&Note app
The Knit&Note app keeps your yarn stash and lets you write notes inside each project, a good place to record the hook you actually used after changing sizes for gauge. The pattern reader explains terms when you tap them, and the row counter keeps the count you tap.
FAQ
What size is an H crochet hook?
An H hook is 5 mm, usually labeled H-8.
What is the most common crochet hook size?
There is no single standard, but hooks from 5 mm (H-8) to 6 mm (J-10) get a lot of use, because they pair with the medium-weight yarns many patterns call for.
Does a higher hook number mean a bigger hook?
For regular US hooks, yes: the letters and numbers go up with the size. Steel thread hooks work the other way, where a higher number means a smaller hook.
Can I use a different hook than the pattern says?
Yes, as long as you still get the pattern's gauge. The hook is a tool for reaching the right fabric. The gauge is what has to match.
