How do crochet hook sizes work?

Metric sizes are the hook diameter in mm. US sizes use letters (B through S) and numbers. UK old sizes run in reverse (highest number = smallest hook). The metric size is the most precise and universally understood.

What size hook for DK yarn?

The Craft Yarn Council recommends a 4.5-5.5mm hook for DK (Light, category 3) yarn, with 5.0mm being the most common starting point — adjust to suit your desired fabric density. For amigurumi, go down 1-2 sizes for tighter stitches. For lacy crochet, go up 1-2 sizes for a more open fabric. Always check the yarn ball band for the suggested hook size.

Are crochet hooks the same sizes as knitting needles?

The metric measurements are the same (a 4mm hook is the same diameter as a 4mm needle). However, US/UK numbering systems are completely different between hooks and needles. Always use the metric size for accurate comparison.

What is the difference between inline and tapered hooks?

Inline hooks (like Susan Bates) have a uniform shaft and pointed tip. Tapered hooks (like Boye) have a rounded tip and throat that narrows. The choice is personal preference; neither affects the size conversion, only how the yarn feeds onto the hook.

Why do patterns list different hook sizes in US and UK?

US patterns list the US letter or number size; UK patterns list the metric size. They refer to the same physical hook. Always verify by checking the metric (mm) size on your hook. This converter helps you match between systems.

KnitCalcs

Hook Size Calculator — Metric, US & UK Crochet Hook Conversion

Convert between metric (mm), US letter, US number, and old UK crochet hook sizes.

Metric Size (mm)Select hook size in millimetres

Yarn WeightOptional — shows whether your hook suits the yarn

Project TypeAdjusts recommendation: amigurumi needs tighter fabric, lacy needs looser

How We Calculate This

This converter uses the standard international crochet hook size equivalency table. Select a metric size to see the equivalent US and old UK sizes.

Common hook sizes by yarn weight

Lace / 2-ply: 2.25-3.5mm regular hook (steel hooks 1.4-1.6mm are used for fine crochet thread, not modelled here)
4-ply / Fingering: 2.25-3.5mm (US B/1-E/4)
DK / 8-ply: 4.5-5.5mm (US 7-I/9)
Aran / 10-ply: 5.5-6.5mm (US I/9-K/10.5)
Chunky / 12-ply: 6.5-9.0mm (US K/10.5-M/13)

Hook ranges follow the Craft Yarn Council Standard Yarn Weight System. Project-type and gauge tweaks below are rules of thumb, not standards values — always swatch and match the gauge stated in your pattern.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard yarn weights and gauge. Results may vary with individual tension, yarn brand, and needle size.