Match the diameter, then check the fabric
The same letter is not always used for the same hook diameter by every manufacturer. Millimetres identify the shaft diameter and are the useful starting point for a substitution. The US letters shown here are common reference labels for ordinary crochet hooks, not a universal manufacturing specification. Steel thread hooks use another sizing system and are not included.
| Diameter | Common US label |
|---|---|
| 3.5 mm | E-4 |
| 3.75 mm | F-5 |
| 4 mm | G-6 |
| 4.5 mm | 7 |
| 5 mm | H-8 |
| 5.5 mm | I-9 |
| 6 mm | J-10 |
| 6.5 mm | K-10½ |
| 8 mm | L-11 |
| 9 mm | M/N-13 |
| 10 mm | N/P-15 |
The same words can mean a different stitch
US double crochet is UK treble crochet. UK double crochet is US single crochet. Confirm which system the designer uses before following a pattern; changing the hook size will not fix a stitch-language mix-up. Chain and slip stitch keep the same names in these two systems.
| US terminology | UK terminology |
|---|---|
| Single crochet (sc) | Double crochet (dc) |
| Half double crochet (hdc) | Half treble crochet (htr) |
| Double crochet (dc) | Treble crochet (tr) |
| Treble crochet (tr) | Double treble crochet (dtr) |
| Double treble crochet (dtr) | Triple treble crochet (trtr) |
| Slip stitch (sl st) | Slip stitch (sl st) |
| Chain (ch) | Chain (ch) |
Length conversions you can check
One yard is exactly 0.9144 metres, and one inch is exactly 2.54 centimetres. A 200 m ball therefore contains about 218.723 yards. Converting a label from metres to yards changes only the unit, not how much yarn is in the ball. Keep the unrounded number until the final calculation when working out several balls.
There is no fixed conversion from grams to yards. For that you need the length and weight printed on the particular yarn label. A 100 g ball containing 200 m has a label ratio of 2 m per gram; a different 100 g ball may have a completely different length.
Reference: Craft Yarn Council standards. Calculations and examples are developed for Loop & Measure; no affiliation or endorsement is implied.
Read the pattern-language guide or learn why yarn weight is not ball weight.