Crochet Hook Size Converter
Results appear instantly after selecting a size. All three systems are shown simultaneously.
Select a Hook Size
Choose your hook type, select the sizing system you know, then pick a size to instantly see all three system equivalents, the yarn weight recommendation, and a size comparison chart.
How to Use This Converter
Choose Your Hook Type
Select 'Standard' for regular aluminum, bamboo, or plastic hooks used with yarn. Select 'Steel (Thread)' if you are working with crochet thread or lace-weight thread and need to look up a steel hook. Steel hooks have a separate size range and use reverse numbering logic.
Select Your Known System
Choose the sizing system printed on your hook or stated in your pattern. Use 'Metric (mm)' if your hook has a millimeter diameter stamped on it. Use 'US Letter/Number' for American patterns (B-1, G-6, H-8, etc.). Use 'UK/Canadian' when working from British or Canadian vintage patterns that use the old number system.
Pick Your Size and Read the Conversions
Select the specific hook size from the dropdown. The converter instantly displays the equivalent size in all three systems, the recommended yarn weight category, and the equivalent knitting needle size. A size comparison bar chart shows where your selected hook falls relative to other common sizes.
Use 'Find Hook by Yarn Weight' for Reverse Lookup
If you know the yarn weight but not the hook size, use the 'Find Hook by Yarn Weight' section. Select a yarn weight category (Lace, Super Fine, Medium, Bulky, etc.) and the tool shows the full recommended hook range in all three systems — useful when you only have a yarn label with a weight category and no pattern.
Frequently Asked Questions
Why does my hook say 'G-6' and '4.0 mm' — which measurement should I trust?
Always trust the millimeter measurement. The Craft Yarn Council, Vogue Knitting, and other industry authorities all recommend relying on metric sizing because letter-number designations vary between manufacturers. A 'G-6' hook from one brand may measure 4.0 mm, while another brand's G-6 measures 4.25 mm. The millimeter diameter is a physical measurement that is independent of any manufacturer's labeling convention. When following a pattern, use the stated mm size as your guide and verify with a gauge swatch. If your hook's markings have worn off, a needle gauge tool — a flat plate with labeled holes — will tell you the actual millimeter diameter.
What is the difference between standard and steel crochet hooks?
Standard hooks (also called aluminum, bamboo, or plastic hooks) are used with yarn — from fingering weight all the way up to jumbo chunky yarn — and range from about 2.0 mm to 25 mm or larger. Steel crochet hooks are much finer, ranging from 0.75 mm (steel #14, the smallest) to 3.5 mm (steel #00, the largest), and are designed for use with crochet thread to create fine lace, doilies, and appliqué. The key difference besides diameter is the numbering direction: standard US hook numbers increase as the hook gets larger, but steel hook numbers are inverted — a steel #14 is smaller than a steel #1. This reversal is a common source of confusion for beginners moving from thread crochet to yarn work or vice versa.
Why do some metric sizes have no US equivalent?
The US letter-number system was not designed to cover every possible millimeter diameter — it evolved incrementally as manufacturers settled on common sizes. As a result, several metric sizes (including 2.0 mm, 2.5 mm, 3.0 mm, 7.0 mm, and 12.0 mm) have no standard US letter-number equivalent. If a pattern specifies one of these sizes, it will typically list only the metric measurement. When purchasing hooks, check the mm label rather than the US size label for these intermediate sizes. Our converter marks these gaps with a dash in the US column and, when you enter one of these measurements, returns the nearest standard sizes above and below with a clear note.
I have a vintage UK pattern that calls for a size 8 hook — what size is that in mm?
A UK size 8 in the old UK/Canadian system corresponds to 4.0 mm (US G-6). This is one of the most commonly encountered UK sizes in vintage patterns from the mid-twentieth century. The UK old-number system runs roughly inverse to diameter at the lower end — UK size 14 is only 2.0 mm, while UK size 3 is 6.5 mm. Note that the system has some gaps: there is no standard UK equivalent for several metric sizes like 3.75 mm (US F-5), and the old system stops being documented for sizes larger than UK 000 (10.0 mm). For large-hook or super-bulky projects in vintage UK books, the millimeter measurement is usually provided alongside the number. Use our UK column in the converter to look up any size from UK 14 down to UK 000.
How do I know which hook size to use if I only have a yarn label?
Most commercially sold yarn labels include a recommended hook or needle size, usually printed as a small crochet hook icon with a size range. This range is the manufacturer's starting recommendation for typical medium-tension crochet. However, the recommended size is a guideline, not a rule. If your swatch shows too many stitches per inch (fabric is too dense), go up one hook size. If your swatch shows too few stitches per inch (fabric is too loose), go down one hook size. Our 'Find Hook by Yarn Weight' feature provides the full standard range for each yarn weight category, so you can see all available sizes in mm, US, and UK formats. Always work at least a 4×4 inch swatch and compare to your pattern's stated gauge before committing to a full project.
Can I use a crochet hook and a knitting needle of the same mm size interchangeably for yarn weight purposes?
Yes — for the purpose of selecting yarn weight and estimating gauge, a crochet hook and a knitting needle of the same millimeter diameter are equivalent. A 5.0 mm crochet hook (US H-8) and a US size 8 knitting needle both have a 5.0 mm shaft diameter and are both recommended for light to medium weight yarn. This cross-compatibility is useful when reading knitting patterns to understand yarn weight, or when shopping for hooks in a store that carries both. The actual gauge you achieve will still differ between knitting and crochet because the stitch structures are different — knitting stitches are generally taller and more open than single crochet stitches. But for yarn weight categorization and hook selection, the millimeter diameter is the shared reference point across both crafts.