Yarn Wraps Per Inch Calculator
Wraps per inch (WPI) is the handspinner's universal measure of yarn thickness: you wrap the yarn snugly around a ruler, count the wraps over a known span, and divide to get wraps per inch. It works across any fiber and lets you match handspun yarn to a pattern's weight category. This calculator turns your counted wraps and measured span into a WPI figure, suggests the matching yarn-weight band, and optionally estimates grist (yards per pound) when you weigh a measured length. Because WPI varies with wrap tension and yarn loft, it is always a measurement of your own sample rather than a fixed constant.
WPI and grist formula
WPI = wraps counted / span measured (inches)
yards per ounce = sample length / sample weight
grist (yards per pound) = yards per ounce * 16
Measuring over a longer span and dividing reduces counting error. There are 16 ounces in one pound, so yards per pound is sixteen times yards per ounce. Grist only appears when you enter both a sample length and weight.
Weight band guideline
- Lace: about 18 or more WPI.
- Fingering: about 14 to 18 WPI.
- Sport: about 12 to 14 WPI.
- Worsted: about 9 to 12 WPI.
- Bulky: below about 9 WPI. These bands are guidelines, so confirm against your pattern gauge.
Yarn wraps per inch: frequently asked questions
What is wraps per inch?
Wraps per inch (WPI) is the number of times a yarn wraps around a one-inch span when laid side by side without crowding or gaps. It is the standard way handspinners and knitters gauge yarn thickness independent of fiber. A higher WPI means a finer yarn; a lower WPI means a thicker yarn.
How do I measure WPI accurately?
Wrap the yarn around a ruler or a WPI gauge over a measured span (often two inches for accuracy), keeping wraps touching but not squished. Count the wraps, then divide by the span length. Measuring over a longer span and dividing reduces counting error.
How is WPI converted to a yarn weight category?
WPI bands map loosely to common yarn weights: lace is roughly 18 or more WPI, fingering around 14 to 18, sport about 12 to 14, worsted near 9 to 12, and bulky below about 9. These bands are guidelines, not a fixed standard, so the calculator reports your WPI and a suggested band you can adjust.
Is WPI an exact constant for a yarn?
No. WPI depends on how tightly you wrap and on the yarn's loft and twist, so it is a measurement that can vary between people and samples. Always measure your own yarn rather than assuming a fixed value. The calculator uses your counted wraps and span, so the result reflects your sample.
What is grist and how does it relate to WPI?
Grist is the yarn's length per unit weight, typically yards per pound. Finer yarns (higher WPI) usually have more yards per pound. This calculator can estimate grist if you weigh a measured length, which is the most reliable way to know a handspun yarn's yardage.
Official sources
- U.S. National Institute of Standards and Technology: Office of Weights and Measures (inch, yard, ounce and pound definitions).
- NIST: National Institute of Standards and Technology (unit conversions).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.