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Crochet thread sizes and the hooks that match them
Crochet thread is numbered by thickness, and the higher the number the finer the thread. Size 10 is the most common and is worked with a 1.5–1.8 mm steel hook; sizes 20 and 30 are finer, for doilies and heirloom lace; sizes 3 and 5 are heavier, closer to a fine yarn.
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What are the crochet thread sizes, and what is each one used for?
Crochet cotton is sold by number. The number counts how many hanks of a set weight make up a pound of thread, so a bigger number means more thread per pound and a finer strand. Sizes 10, 20 and 30 cover most modern thread patterns; anything finer than 40 is specialist and sold mainly for lace and tatting.
The hook column below is a starting range rather than a rule. Thread brands differ in how tightly they are spun, and tension differs between crocheters, so swatch and match the gauge your pattern prints.
| Thread size | Typical hook | Typical projects | Availability today |
|---|---|---|---|
| 3 | 2.5–3.0 mm | Bags, baskets, placemats, chunky lace | Widely sold; the heaviest crochet cotton. |
| 5 | 2.1–2.5 mm | Coasters, small bags, bold doilies | Widely sold, including as pearl cotton 5. |
| 10 | 1.5–1.8 mm | Doilies, edgings, bookmarks, appliqués | Widely sold; the most common thread today. |
| 20 | 1.25–1.5 mm | Fine doilies, collars, handkerchief edgings | Widely sold. |
| 30 | 1.1–1.3 mm | Heirloom doilies and filet lace | Sold by most thread brands. |
| 40 | 1.0–1.15 mm | Fine lace, tatting, insertions | Sold by lace and tatting brands. |
| 50 | 0.9–1.0 mm | Very fine lace and Irish crochet | Specialist; tatting and heirloom ranges. |
| 70 | 0.85–0.9 mm | Antique-style lace edgings | Specialist; limited colours. |
| 80 | 0.75–0.85 mm | Fine antique lace, doll clothes | Specialist; usually white and ecru only. |
| 100 | 0.6–0.75 mm | Museum-fine lace and miniatures | Rare; a few heirloom cotton ranges. |
| 150 | 0.5–0.6 mm | Miniature lace; rarely called for | Very rare today; size 100 is the usual substitute. |
What size hook do you use with size 10 crochet thread?
Size 10 crochet thread is usually worked with a steel hook between 1.5 mm and 1.8 mm, which is US steel size 8, 7 or 6. Go down to 1.3 mm for tight, crisp lace, or up to 2 mm for a softer fabric with more drape.
Size 10 is the thread most patterns assume when they simply say "crochet cotton". It is the easiest size to find in colours, it shows stitch definition clearly, and it is the size to start with if you have never worked thread crochet before.
What are the modern US steel crochet hook sizes in mm?
Steel hooks are numbered in the opposite direction to letter hooks: the higher the steel number, the finer the hook. The millimetres below are the conversions used by steel hooks sold today.
Hook numbers printed in vintage pattern books were never standardised between makers, so an old pattern's hook number is a guide only. Choose the hook that gives the printed gauge.
| US steel size | Millimetres |
|---|---|
| 00 | 3.5 mm |
| 0 | 3.25 mm |
| 1 | 2.75 mm |
| 2 | 2.25 mm |
| 3 | 2.1 mm |
| 4 | 2 mm |
| 5 | 1.9 mm |
| 6 | 1.8 mm |
| 7 | 1.65 mm |
| 8 | 1.5 mm |
| 9 | 1.4 mm |
| 10 | 1.3 mm |
| 11 | 1.1 mm |
| 12 | 1 mm |
| 13 | 0.85 mm |
| 14 | 0.75 mm |
Can you substitute one crochet thread size for another?
Yes, as long as you accept a different finished size. Working a pattern in a heavier thread makes the piece bigger, and a finer thread makes it smaller; the stitch count stays the same. Change the hook to suit the new thread, then work the first few rounds and measure before committing.
This matters most with antique patterns, which often call for sizes 70, 80 or 100 that are hard to buy now. A doily written for size 100 worked in size 20 is still the same design, just several inches wider.
Is crochet thread the same as yarn?
Crochet thread is a tightly twisted, usually mercerised cotton that is thinner and firmer than yarn, so it holds lace open and keeps a blocked shape. Yarn is softer and loftier, and is measured by weight category rather than by thread number.
Size 3 thread is the point where the two overlap: it is close to a fingering or sport yarn in thickness, but still firmer. If a pattern calls for size 3 and you only have fine cotton yarn, swatch first — the drape will differ even when the gauge matches.
Patterns to practise on
How to crochet a classic granny square
- Skill level
- Beginner
- Yarn weight
- Medium (#4)
- Hook size
- 5 mm hook
- Yardage
- 35 yd
Free pattern
Easy moss stitch crochet dishcloth
- Skill level
- Beginner
- Yarn weight
- Medium (#4)
- Hook size
- 5 mm hook
- Yardage
- 100 yd
Free pattern
Simple half double crochet baby beanie in the round
- Skill level
- Beginner
- Yarn weight
- Medium (#4)
- Hook size
- 5 mm hook
- Yardage
- 130 yd
Free pattern
Quick round crochet coaster in cotton
- Skill level
- Beginner
- Yarn weight
- Medium (#4)
- Hook size
- 4.5 mm hook
- Yardage
- 18 yd
Free pattern
Frequently asked questions
Which crochet thread size is thickest?
Size 3 is the thickest crochet cotton in common sale, followed by size 5. The numbers run backwards: size 100 is far finer than size 10.
What thread size do most doilies use?
Size 10 for everyday doilies, size 20 or 30 for finer heirloom pieces. Older patterns often call for size 30 to 70.
What is size 10 crochet thread in yarn weight?
It is roughly a lace-weight (category 0) cotton, but tightly twisted and firmer than lace yarn, so it behaves differently in the same stitch pattern.
Can you crochet thread patterns with an aluminium hook?
Only the heavier sizes. Below about 2 mm, hooks are made in steel because aluminium is too soft to hold a fine, sharp head.