Why do US and UK crochet hook numbers not match?

The US and UK systems evolved independently over the 20th century and use different numbering conventions. US hooks use a letter-plus-number code (B-1, G-6, H-8, etc.) while UK hooks use a descending plain number scale where larger numbers mean smaller hooks — the opposite of US. Metric millimetres became the international standard from the 1980s onward and are now printed on almost all modern hooks; they are the only truly unambiguous way to specify a size.

What is the most common crochet hook size?

For standard yarn-weight projects the most-used sizes are 4.0 mm (US G-6 / UK 8), 5.0 mm (US H-8 / UK 6) and 5.5 mm (US I-9 / UK 5). A 5.0 mm hook suits most double-knit (DK) yarn and is a good starting point for beginners.

Are UK crochet hook sizes still used?

The UK numerical system has largely been replaced by metric sizing on modern hooks sold in the UK, Europe and internationally. However, older patterns — especially those published before the 1990s — may still use UK numbers, and vintage hooks in craft collections are often stamped with the old UK scale. This converter covers all sizes including the discontinued steel hook range.

What size hook should I use for a specific yarn weight?

Yarn weight and hook size are matched by the recommended gauge on the yarn label. As a rough guide — 2.0–2.5 mm for lace, 3.0–3.5 mm for fingering/sock, 3.5–4.5 mm for DK, 5.0–6.0 mm for worsted/aran, 6.0–9.0 mm for bulky, and 10 mm or above for super-bulky and jumbo yarns. Always work a tension swatch and adjust hook size until your stitch count matches the pattern.

What does the mm measurement on a crochet hook refer to?

The millimetre figure is the outer diameter of the hook shaft, measured at its widest circular cross-section. This is the dimension that determines how large each stitch loop will be. The head shape, throat depth and handle length vary between brands but do not affect the stated mm size.

Can I use a knitting needle instead of a crochet hook?

In an emergency a straight knitting needle in the same mm diameter will work for simple chain and single crochet stitches, but the lack of a hook head makes most stitches very awkward. Tunisian crochet specifically requires an elongated hook or knitting needle, so that is the one case where a needle substitution is practical.

Crochet Hook Size Converter

A crochet hook size converter covering every size from 0.6 mm steel lace hooks up to 25 mm jumbo hooks — with instant three-way translation between the US letter system, the UK number system and the universal metric (mm) standard. Pick your starting system, enter a size, and all three equivalents appear side-by-side. An expandable full reference table lists every standard size so you can browse the whole scale at once.

How it works

There is no mathematical formula connecting hook sizes — the US and UK systems were assigned historically and both map onto a fixed lookup table tied to the physical millimetre diameter of the hook shaft. The converter stores the full Craft Yarn Council (US) and legacy British Standard (UK) tables and does a direct lookup based on the selected mode:

Metric input: the entered mm value is snapped to the nearest standard size in the table.
US input: the converter finds every row that carries that US label and returns the first matching mm and UK value.
UK input: the converter finds every row with that UK number.

Because some US labels span two metric sizes (for example US D-3 covers both 3.0 mm and 3.25 mm) the table preserves both rows so no size is lost.

Worked example

You have an old UK pattern calling for a size 8 hook. Select “UK size”, choose 8 from the dropdown, and the tool shows:

Metric	US	UK
4.0 mm	G-6	8

You walk into a modern craft shop, ask for a 4 mm or G-6 hook, and get the right size — regardless of which country the hook was manufactured in.

Conversely, if a US pattern specifies an H-8, the converter returns 5.0 mm / UK 6. A quick check: the mm size printed on any modern hook should match the table value.

Why three systems exist

The metric system is printed on virtually all hooks sold worldwide today and is the unambiguous standard. The US letter-and-number codes (B-1 through S and beyond) are widely used in North American patterns and on hooks sold in the United States and Canada. The UK numeric scale — where larger numbers = smaller hooks — appears in British patterns published before the 1990s and is still encountered on vintage hooks. Steel lace-hook sizes (US “Steel” suffixed) have no UK equivalents and are used exclusively for thread crochet; they are included in the full table for completeness.

Reference: yarn weight guide

Yarn weight	Typical hook range
Lace / thread	0.75–1.75 mm
Fingering / sock	2.0–2.5 mm
Sport	2.75–3.5 mm
DK	3.5–4.5 mm
Worsted / aran	4.5–5.5 mm
Bulky	6.0–9.0 mm
Super bulky	10–15 mm
Jumbo	16 mm and above

Always check the tension swatch — yarn fibre, twist and personal tension all affect the effective gauge even when the hook size matches.