# Crochet Hook Converter

> Crochet hook converter: translate hook sizes between US letters, UK numbers, and metric millimeters for any yarn pattern. Free, private, runs in your browser.

- **Canonical URL:** https://boringtoolskit.com/crochet-hook-converter/
- **Category:** Converters
- **Evidence level:** authored-methodology
- **Reviewed:** 2026-09-20
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

US letter sizes map to mm (B = 2.25 mm ... K = 6.5 mm) with the same no-formula convention as knitting needles; metric sizing is the primary standard.

## Assumptions

- Manufacturer charts vary; conversion outputs are the standard-mapping midpoint.
- Measurements are body measurements unless the tool labels them as garment measurements.
- Between-size results round per the rule stated in the FAQ.

## Common questions

### Why do patterns give both letters and mm?

US patterns use letters, European patterns use mm. Gauge again decides: match the mm the pattern's gauge assumes, and treat the letter as its nickname.

## Sources

- [Aggregated manufacturer sizing conventions](https://boringtoolskit.com/methodology/)

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