Found a mystery skein with no label? Wraps per inch (WPI) tells you its weight. Wrap the yarn snugly around a ruler, count the wraps in one inch, and this tool names the yarn weight, the projects it suits, and what it can substitute for — lace through super bulky.
WPI by yarn weight
| Weight | WPI | Typical projects |
|---|---|---|
| Lace | 20+ | Lace shawls, doilies, fine wraps |
| Fingering | 16–19 | Socks, shawls, baby items |
| Sport | 14–15 | Light sweaters, baby clothes |
| DK | 12–13 | Sweaters, cardigans, accessories |
| Worsted | 9–11 | Hats, scarves, blankets, sweaters |
| Bulky | 6–8 | Quick hats, cowls, throws |
| Super Bulky | 5 or fewer | Chunky blankets, arm-knit projects |
WPI charts vary slightly between sources; treat the result as a confident starting point and confirm with a gauge swatch.
How to measure WPI
- Wrap the yarn around a ruler or a WPI tool, filling exactly one inch.
- Lay the wraps side by side — touching but never overlapping.
- Keep tension snug but relaxed; do not stretch or compress the strand.
- Count the wraps in that inch. For accuracy, wrap over 2 inches and divide by two.
Fibre behaviour & WPI accuracy
- Fluffy, lofty yarns (singles, roving) compress under wrapping, so they can read finer than they knit up — wrap with a light hand.
- Halo yarns like mohair and brushed alpaca have fuzz that distorts the count; wrap and count the core strand, not the halo.
- Smooth, inelastic fibres (cotton, linen, silk) give the truest reading; springy wool can read a little off if you pull tight.
- When a reading sits on the border between two weights, knit a quick swatch to settle it.
Unknown yarn recovery
Every crafter eventually becomes a forensic yarn investigator. For an unlabelled, inherited, or thrifted skein:
- WPI is the fastest way to place mystery yarn on the weight scale — no label required.
- Wrap a short length you can spare; you do not need to unwind the whole skein.
- Confirm with a quick gauge swatch on the suggested needle or hook size.
- Write the result on a tag so the same skein never stumps you twice.
Why WPI matters
- Estimates gauge — gives you a needle size to start from before swatching.
- Powers substitutions — matching WPI is a reliable way to swap one yarn for another.
- Identifies mystery yarn — the go-to method when there is no ball band.
- Hints at drape and yardage — thickness drives how a fabric hangs and how far a skein goes.
Frequently asked questions
What is WPI (wraps per inch)?
WPI counts how many times a yarn wraps around a ruler within one inch. Thinner yarn wraps more times, making it a quick measure of yarn thickness and weight.
How do I measure wraps per inch?
Wrap the yarn snugly without stretching around a ruler, lay the wraps side by side with no gaps or overlaps, and count how many fit in one inch. Wrapping over two inches and dividing improves accuracy.
How does WPI tell me the yarn weight?
More wraps means finer yarn. Roughly 20 or more is lace, 16 to 19 fingering, 14 to 15 sport, 12 to 13 DK, 9 to 11 worsted, 6 to 8 bulky, and 5 or fewer super bulky.
Can WPI help me substitute yarn?
Yes. Matching the WPI of two yarns is one of the most reliable ways to confirm they are the same weight before swapping one into a pattern.
Is WPI more reliable than the ball band?
It is invaluable when there is no band. When you have a label, use WPI to confirm it, since weight names are not standardised between brands.