Crochet Hook Size Converter - US Letter, Metric and UK Hook Sizes

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Why crochet hooks have three numbering systems

Crochet hooks are labelled in millimetres, in US letters, and in UK numbers, and the three systems agree only loosely. A 5 millimetre hook is a US H-8 and a UK 6. A 6 millimetre hook is a US J-10 and a UK 4. The UK numbers run backwards, so a smaller UK number means a thicker hook, while US letters run forwards from B to P and beyond. Add steel hooks, which use yet another numbering for thread crochet, and it is no wonder that a pattern from another country can stop a crocheter at the materials list.

This converter handles the three main systems with unit toggles. Tap mm, US or UK, pick your size, and the tool shows the matching size in all three systems. If your size has no exact match in one system, the tool shows the nearest size and says so, because some sizes simply do not exist everywhere. UK hook numbers also vary slightly by maker, so the tool tells you to check the maker’s own chart before buying.

How to use the converter

Tap the button for the system your pattern or hook uses: mm, US or UK.
Pick the size from that system’s list.
Read the matching sizes in the other two systems.
If the result is a nearest match, swatch with the substitute hook before starting a large project.

The table covers the hooks most crocheters own, from fine 2.25 millimetre hooks up to chunky 15 millimetre hooks. Thread hooks below 2 millimetres use steel sizes and are outside this table, because their numbering follows a different tradition.

A worked example

Suppose an American pattern calls for a US H-8 hook, but your hooks are labelled in millimetres. Tap US, pick H-8, and the converter shows 5.0 millimetres and UK 6. Suppose instead a vintage British pattern asks for a UK 4. Tap UK, pick 4, and the tool shows 6.0 millimetres and US J-10. Both patterns are now usable with the hooks you already own.

Try a size with no exact match: 4.5 millimetres has a US size of 7 but no UK equivalent in the table. The tool shows that plainly instead of inventing a UK number. A crocheter who buys a UK 6 or 7 as a substitute and swatches first will still succeed, but one who assumes an exact match exists will blame the pattern when the gauge is off.

Tips for choosing hooks

Trust millimetres first. They are a real measurement, while letters and UK numbers are labels that hide rounding between makers.
Keep a hook gauge card with your tools. It measures any unlabelled hook in seconds.
Remember that hook material changes how yarn slides. The same size in bamboo, aluminium and ergonomic plastic can feel like three different hooks, and your tension will follow the feel.
When a pattern gives a hook size and a gauge, trust the gauge. Hook labels vary, but stitches per inch do not lie.

Common mistakes

The most common mistake is confusing US and UK numbers. A UK 6 is a 5 millimetre hook, while a US 6 is a 4 millimetre hook, and swapping them changes the fabric completely. Another common mistake is assuming a letter hook and a numbered hook are the same family: US letters cover yarn hooks, while steel thread hooks use small numbers, and the two families overlap confusingly around size 6.

A third mistake is buying a full hook set for one pattern. Most crocheters use a handful of sizes regularly. Buy the size the pattern needs, swatch, and let the rest of the set earn its place over time.

Who needs a hook converter

Anyone crocheting from patterns written in another country needs one, anyone inheriting hooks or buying second-hand sets needs one, and anyone shopping online where the listing uses an unfamiliar system needs one. Hook conversion is a tiny task, but getting it wrong wastes yarn and patience, and this tool makes it take seconds with the unit toggles doing the switching for you.

Crochet Hook Size Converter – US Letter, Metric and UK Hook Sizes