Instantly convert between US Letters, UK numbers, and Metric (mm) sizes for all regular and steel lace crochet hooks.
Look up any US Letter or Metric (mm) size instantly
Complete conversion for aluminum, wood, and plastic crochet hooks used for standard yarn weights:
| US Size | Metric (mm) | UK / Canada | Recommended Yarn Weight | Typical Projects |
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Steel hooks use a reverse numerical numbering system where higher numbers represent smaller hook diameters:
| Steel Hook No. | Metric (mm) | Best Crochet Thread Size | Typical Application |
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Crochet hook sizing can be notoriously confusing because different countries and eras used completely distinct systems. While modern international patterns almost universally specify the shaft diameter in Metric millimeters (mm), vintage patterns and US labels often rely on letters or numbers.
The US system primarily utilizes letters from B (2.25 mm) through S (19 mm), frequently paired with numbers (like G-6 or H-8). In contrast, the traditional UK and Canadian systems utilized inverted numbers where smaller numbers indicated larger hooks. To guarantee your crochet project hits the exact gauge swatch measurements, always look at the metric millimeter size stamped on your hook shank.
If you inherited vintage crochet hooks with worn-off labels, never guess the size. Insert the cylindrical shaft (the straight portion between the hook head and thumb grip) into a standard knitters' needle gauge ruler. The smallest hole through which the shaft slides through snugly without bending is your exact millimeter size.
A 5.0 mm crochet hook is a US size H-8 (often referred to simply as size H). In the UK, it is known as size 6. It is the universally recommended starting hook for medium/worsted weight yarn projects.
A standard US size G-6 is 4.0 mm (UK size 8). While a few legacy American brands produced 4.25 mm hooks labeled as G, 4.0 mm is the official craft standard.
Beginners should almost always start with a 5.0 mm (US H-8) or 5.5 mm (US I-9) hook paired with a smooth, light-colored worsted weight acrylic yarn. Stitches are large enough to see clearly without splitting.
Metric millimeter measurements are identical across both craft forms (5.0 mm hook = 5.0 mm needles). However, US letter/number designations diverge significantly! To compare knitting needles, view our dedicated Knitting Needle Size Chart.
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