Crochet Hook Size Chart — mm to US Sizes

Identify an Unmarked Hook

A crochet hook size chart in mm to US sizes only helps once you know which hook you are holding. Calibrate once, then hold the shaft against the circles below — the one it fills exactly is your size. The full crochet hook conversion chart is underneath.

Calibration 96 PPI

Hold a bank card against the outline and drag until the widths match — every card is 85.6 mm wide.

Hook shaft, actual size

Sizes above 12 mm are omitted from the gauge — they are easy to read on a ruler and would dominate the page. They are all in the table below.

Crochet Hook Size Chart: mm to US

This crochet hook size conversion chart gives crochet hook sizes in mm to US letters and numbers. Millimetres are the reliable measurement. US letters and numbers are conventions that vary between manufacturers, and several common sizes have no assigned letter at all. Where a chart and a hook disagree, trust the millimetre marking on the hook.

Crochet hook conversion, millimetres to US size
Millimetres	US size	Typical use
2.25 mm	B-1	Thread, fine lace
2.5 mm	—	Thread, fine lace
2.75 mm	C-2	Thread, fine lace
3.13 mm	D	Fingering, sock yarn
3.25 mm	D-3	Fingering, sock yarn
3.5 mm	E-4	Fingering, sock yarn
3.75 mm	F-5	Fingering, sock yarn
4 mm	G-6	DK, light worsted
4.25 mm	G	DK, light worsted
4.5 mm	7	DK, light worsted
5 mm	H-8	DK, light worsted
5.25 mm	I	DK, light worsted
5.5 mm	I-9	Worsted, aran
5.75 mm	J	Worsted, aran
6 mm	J-10	Worsted, aran
6.5 mm	K-10½	Worsted, aran
7 mm	—	Bulky
8 mm	L-11	Bulky
9 mm	M/N-13	Bulky
1 mm	N/P-15	Super bulky, jumbo
11.5 mm	P-16	Super bulky, jumbo
12 mm	—	Super bulky, jumbo
15 mm	P/Q	Super bulky, jumbo
15.75 mm	Q	Super bulky, jumbo
16 mm	Q	Super bulky, jumbo
19 mm	S	Super bulky, jumbo
25 mm	T/U/X	Super bulky, jumbo
3 mm	T/X	Super bulky, jumbo

Source: Craft Yarn Council, verified 5 August 2026. A dash means the size has no assigned US letter.

How to Measure a Crochet Hook

To convert crochet hook size to mm yourself, measure the shaft — the straight section between the hook head and the thumb rest. Not the head, which is wider, and not the handle, which is wider still. The shaft is what the yarn wraps around, so it is what sets your stitch size, and it is what the printed number refers to.

Calibrate the gauge above against a bank card. This takes ten seconds and is saved.
Hold the hook flat against the screen, shaft across the circles.
Find the circle the shaft covers exactly — not the one it sits inside.

Below 4 mm the gap between adjacent sizes is often 0.25 mm, which is genuinely hard to judge on a ruler. Comparing against a circle of known diameter is more forgiving, because you are matching an area rather than reading a scale.

Why Hook Letters Disagree Between Brands

The Craft Yarn Council publishes recommended letter-to-millimetre pairings, but they are recommendations. Manufacturers are free to deviate and some do — you will find hooks marked "G" at both 4 mm and 4.25 mm, and "Q" at 15.75 mm and 16 mm. Older hooks and imports follow their own conventions.

Three sizes in common circulation — 2.5 mm, 7 mm, and 12 mm — have no assigned US letter whatsoever. If a chart gives you one, it invented it.

Older UK numbering makes this worse by running backwards: a higher UK number means a smaller hook. That system is legacy and was inconsistently applied even when it was current, so this page deliberately does not publish a UK conversion column. Measuring the shaft is more reliable than any table.

Which Hook Size for Which Yarn

A starting point, not a rule — your tension decides the final answer:

Lace / thread — 1.5 to 2.25 mm
Fingering, sock — 2.25 to 3.5 mm (US B-1 to E-4)
DK, light worsted — 3.5 to 4.5 mm (US E-4 to 7)
Worsted, aran — 5 to 6.5 mm (US H-8 to K-10½)
Bulky — 6.5 to 9 mm (US K-10½ to M/N-13)
Super bulky, jumbo — 9 mm and up

Always work a gauge swatch before a garment. Two people using the same hook and the same yarn routinely produce fabric that differs by a full size, because tension is personal. If your swatch has too many stitches per 10 cm, move up a hook size; too few, move down.

Accuracy and Limits

The gauge is as accurate as your calibration. Calibrated carefully against a card it is typically within a few tenths of a millimetre — enough to separate adjacent hook sizes. Two things degrade it:

Browser zoom. Calibrate at 100%, and recalibrate if you change it.
Viewing angle. Look straight at the screen. Judging a circle edge at an angle introduces parallax.

This chart covers standard yarn hooks. Steel thread hooks use a separate numbering system running roughly 0.6 mm to 3.5 mm, where higher numbers mean smaller hooks — a steel 7 and an aluminium 7 have nothing to do with each other. If your hook is very fine and marked with a number above 10, it is a steel hook and this table does not apply.

Frequently Asked Questions

How do you tell what size a crochet hook is?

Measure the shaft — the straight section between the hook head and the thumb rest — not the head or the handle. The shaft diameter is what determines your stitch size, and it is the number stamped on a marked hook. Use the calibrated gauge above: hold the shaft against the circles until you find the one it fills exactly.

How do I measure a crochet hook without a gauge?

Hold the shaft against a millimetre ruler and read the diameter, or lay it beside a hook you have already identified. A ruler is awkward for anything under 4 mm because the difference between adjacent sizes is often 0.25 mm. The on-screen gauge above is easier for the small sizes, provided you calibrate it first.

What size is a 2.5 mm crochet hook?

2.5 mm sits between US B-1 (2.25 mm) and US C-2 (2.75 mm) and has no assigned US letter in the Craft Yarn Council standard. Hooks marked 2.5 mm are common in European and Japanese brands. If a pattern calls for a US B or C and you only have 2.5 mm, work a gauge swatch before committing.

Does crochet hook size matter?

Yes — hook size is the main control over stitch size, and therefore over the finished dimensions of your work. Going up one size loosens the fabric and makes the piece larger; going down tightens it. That said, the size on the label matters less than the gauge you actually achieve, because tension varies from person to person.

Why do crochet hook letters vary between brands?

Because the letters are conventions, not a standard. The Craft Yarn Council publishes recommended letter-to-millimetre pairings, but manufacturers are free to deviate, and older or imported hooks often do. Some sizes — 2.5 mm, 7 mm, 12 mm — have no assigned letter at all. This is why every reliable chart tells you to trust the millimetre marking.

What size crochet hook should I use for DK yarn?

DK weight (Craft Yarn Council weight 3, light) is normally worked with a 3.5 mm to 4.5 mm hook — US E-4 through US 7. Start with 4 mm, make a gauge swatch, and adjust: go up a size if your fabric feels stiff, down a size if it looks loose and gappy.

Can I use a different size crochet hook than the pattern says?

Yes, and often you should. The pattern's hook size is the designer's starting point for their tension, not a rule. What matters is matching the stated gauge — the number of stitches and rows per 10 cm. If your swatch has too many stitches, go up a hook size; too few, go down.

Are US and UK crochet hook sizes the same?

No, and the older UK numbering runs in the opposite direction — a higher UK number means a smaller hook. UK sizing is legacy and inconsistently applied across manufacturers, which is why this chart gives millimetres and US sizes only. If you have a UK-marked hook, measure the shaft rather than trusting a conversion table.

What is the difference between a steel and an aluminium crochet hook size?

Steel hooks use a completely separate numbering system for thread crochet, running from about 0.6 mm to 3.5 mm, where higher numbers mean smaller hooks. An aluminium size 7 and a steel size 7 are nothing alike. The chart on this page covers standard yarn hooks; steel thread hooks are a different scale entirely.