Crochet Hook Converter
Translate a crochet hook size between the US letter system, the UK numbering system and metric millimetres, and see what each gauge is typically used for.
One Gauge, Three Naming Systems
Hook sizes are named differently depending on where a pattern was published, and the same physical hook can carry a letter, a number and a millimetre measurement. The page keeps all three in step from a single lookup table.
- The table holds twenty-one hooks, running from 2.25 mm up to 16.0 mm. Each row pairs a US letter, a UK number and a metric figure.
- Choosing from any one of the three dropdowns updates the other two and prints a plain-language note about what the gauge is for.
- The metric lookup matches on the millimetre value with a tolerance of one-hundredth of a millimetre, which is what lets a decimal value like 3.75 find its row.
- The US system runs from B to U and skips several letters. There is no standard US letter for the 12.0 mm diameter, so that row is listed without one; the Q letter belongs to the 11.5 mm hook alone, which keeps every letter mapped to a single row.
- The recommended-use notes are broad bands rather than rules. Everything at 5.0 mm and above is labelled for afghans and scarves, from 3.75 to 4.5 mm for baby items and socks, and the smallest gauges for lace and fine thread.
Converting a Size
- Open the page and read the default line, which resolves to US B, UK 1 and 2.25 mm, described as a lace and fine-thread gauge.
- If your pattern quotes a US letter, choose it in the US dropdown. The UK and metric selectors follow automatically.
- If your pattern is British, choose the UK number instead and the other two follow.
- If you are holding an unmarked hook, measure it and pick the nearest millimetre value from the metric dropdown.
- Use the Reset button to clear the result panel and return all three selectors to the first entry.
Sizes Through the Table
The lookups below are exactly what the page resolves from its table, covering a normal conversion and the one diameter that has no US letter.
- The default is US B, which the page pairs with UK 1 and 2.25 mm, and it notes lace and fine thread as the typical use.
- The familiar H hook maps to UK 8 and 5.0 mm, at which point the note switches to afghans and scarves, and the same note carries right up to 16.0 mm at the top of the table.
- Selecting Q from the US dropdown returns UK 16 and 11.5 mm, the one hook that carries that letter, and the note reads afghans and scarves.
- The 12.0 mm hook has no US letter, so it appears in the US dropdown as a dash. Choosing it returns 12.0 mm and UK 17, the same row you reach by picking 12.0 mm from the metric list.
- Because each letter now names a single row, every option in the US dropdown resolves to exactly the millimetre value printed beside it, and 11.5 and 12.0 stay distinct in the metric list.
Other Ways to Identify a Hook
| Alternative | Notes |
|---|
| Read the pattern's notation | The most reliable source is the pattern itself. If it states one measurement system, work in that system and ignore the others; converting can introduce drift that the designer never intended. |
| Measure the shaft | For an unmarked hook, measure the shaft's diameter in millimetres and match it to the nearest row. Measuring is more dependable than guessing from the letter, because letters vary a little between manufacturers. |
| Use the gauge swatch | The ultimate check is the swatch your pattern asks for. A hook conversion is a starting point; if the swatch comes out the wrong size, changing hook is the usual fix regardless of what the letter claims. |
| Keep a personal chart | Because old UK numbering and modern US letters do not line up neatly, many crocheters keep their own note of which hook in their collection corresponds to which pattern convention. That personal mapping beats any generic table on a shelf. |
A Conversion Chart in the Page
The table is embedded in the page, so it works without a lookup service and without sending anything.
- Every conversion is a local table search. The page does not query a server to translate a size.
- There is no account and nothing to submit; choosing a value simply rewrites the two neighbouring selectors and the note.
- Once loaded, the page works offline, which is useful in a craft room away from a reliable connection.
- Only your theme preference is stored locally between visits; the selected hook is not remembered.
- The notes are fixed text baked into the page, not fetched, so they display even with no connection at all.
Crochet Hook Questions
Why is one row listed without a US letter?
Because 12.0 mm has no standard US letter — the letters are a manufacturer convention, not a specification, and none of them is defined for that diameter. Q is the 11.5 mm hook, and S is 14.0 mm, leaving a gap at 12.0 mm. The page shows that row with a dash in the US dropdown and lets you reach it through the millimetre value instead, which is how such a hook is usually sold.
Are UK numbers the same as US letters?
No, and they do not map in any simple order. UK sizes are numbers, from 1 upwards, while US sizes are mostly letters. The same physical hook can be UK 8 and US H, which is why pattern conversions go wrong when someone assumes the numbers and letters are equivalent.
How accurate is the millimetre column?
It is a standard reference table, and the rows are spaced to match common commercial hooks. Real hooks vary slightly by brand, so a 4.0 mm hook from one maker may measure marginally differently from another's. Use the table to find the right neighbourhood, then confirm with a gauge swatch for anything where size matters.
What is each hook size used for, broadly?
The page groups gauges into rough uses: the smallest, around 2.25 to 3.5 mm, for lace and fine thread; 3.75 to 4.5 mm for baby items and socks; and 5.0 mm and up for afghans and scarves. These are conventional pairings with typical yarn weights rather than hard limits, and the right hook is ultimately the one that gives you the fabric the pattern expects.
Last verified: October 3, 2026
Launch Crochet Hook Converter
All Tools · All Tools · About · How We Test · Contact · Privacy
Maintained by Huang Bin · Last updated October 3, 2026