Tools
WPI (Wraps Per Inch) & Mystery Yarn Identifier
Found a ball of yarn without its label? Count how many times it wraps around a ruler in one inch, or weigh a measured sample, and the identifier tells you its official Craft Yarn Council weight category, the technical name, the crochet hook range, the base gauge in 10 × 10 cm and the five commercial yarns in our database with the closest specifications.
Free · No sign-up · Runs in your browser
- CYC category
- #4 Medium
- Technical name
- Worsted · Aran · Afghan
- Crochet hook
- 5.5–6.5 mm (US I-9 – K-10½)
- Base gauge 10 × 10 cm
- 11–14 single crochet
| Your measurement | 10 wraps per inch |
|---|---|
| Estimated linear density | ≈ 170 m / 100 g · 186 yd / 100 g |
| Tex · metric count | ≈ 588.2 tex · Nm 1.7 |
| Official CYC WPI range | 9–12 WPI |
| Regional names | Worsted (US) / Aran (UK) / 10-ply (AU/NZ) |
Closest Commercial Yarn Matches
- Lion Brand 24/7 Cotton100 % density match
#4 Medium · 170 m / 100 g · 170 m per 100 g ball
- Lion Brand Basic Stitch Anti-Pilling100 % density match
#4 Medium · 170 m / 100 g · 170 m per 100 g ball
- Paintbox Yarns Cotton Aran100 % density match
#4 Medium · 170 m / 100 g · 85 m per 50 g ball
- Rico Design Creative Cotton Aran100 % density match
#4 Medium · 170 m / 100 g · 85 m per 50 g ball
- Scheepjes Cahlista100 % density match
#4 Medium · 170 m / 100 g · 85 m per 50 g ball
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Assumptions: the official CYC WPI ranges overlap, so we assign one category with intermediate cut-offs (above 30, 18.5, 14.5, 11.5, 8.5, 6.5 and 4.5 WPI); density cut-offs are the geometric means of our calculator’s reference densities (800, 400, 300, 230, 170, 110, 65 and 25 m per 100 g), and WPI is converted to density by interpolating between them. Sources: Craft Yarn Council (Standard Yarn Weight System, Wraps Per Inch) and ISO 1144 (tex).
How it works: the math behind the tool
Wraps per inch. Wrap the yarn around a ruler or a pencil for a few inches, snug but without stretching it, with every wrap lying next to the previous one and no gaps or overlaps, then count the wraps that fit in one inch (2.54 cm) in several places. The Craft Yarn Council publishes a WPI range for each category: 30–40+ for #0 Lace, 14–30 for #1 Super Fine, 12–18 for #2 Fine, 11–15 for #3 Light, 9–12 for #4 Medium, 6–9 for #5 Bulky, 5–6 for #6 Super Bulky and 1–4 for #7 Jumbo. Those ranges overlap, so the identifier assigns one category with intermediate cut-offs and warns you when your reading also falls inside the official range of the neighbouring category.
Linear density. WPI depends on how tightly you wrap, so the precise method is to measure 10 m or more of yarn, weigh that sample on a 0.1 g scale and divide: metres ÷ grams × 100 gives metres per 100 g. The same figure is expressed as tex (grams per 1,000 m, the universal system of ISO 1144) and as the metric count Nm (metres per gram). The category comes from the reference densities our yarn calculator uses for each CYC weight, with the boundary between two categories placed at their geometric mean. If you also weigh the whole ball, the tool estimates how many metres are left in it.
Density (m/100 g) = sample m ÷ sample g × 100 · tex = 100,000 ÷ density · Nm = density ÷ 100 · Category = official CYC WPI range or density cut-off · Match = |ln(density ratio)| + CYC and fiber penalties
Closest commercial yarns. The measured or estimated density is compared with every current yarn in the CrochetMetrics database. Each yarn is ranked by the logarithmic distance between densities, with a penalty when it belongs to another CYC category and another when it does not match the strand structure you selected (plied cotton, fluffy acrylic, smooth merino or chenille and velvet). The five best matches show their density match percentage, a link to the full specification sheet and buttons to Amazon and partner stores. In WPI mode the density is interpolated between the reference yarns, so weigh a sample whenever you need an exact replacement.
Last updated: November 8, 2026
Frequently asked questions
How do I measure wraps per inch?
Wrap the yarn around a ruler or pencil so the strands lie side by side without gaps, overlaps or stretching, and count how many wraps fill one inch (2.54 cm). Check several spots and use the average: the Craft Yarn Council notes that WPI is subjective and depends on wrapping tension.
What WPI is worsted weight yarn?
The Craft Yarn Council range for #4 Medium (worsted, aran and afghan yarns) is 9 to 12 wraps per inch. Because 11 and 12 WPI also fall inside the #3 Light range, a reading there should be confirmed with a swatch or by weighing a sample.
How can I identify yarn without a label?
Measure wraps per inch for a quick category, or weigh 10 m of yarn on a precision scale to calculate metres per 100 g. Then compare the strand structure and fiber feel: the identifier lists the five commercial yarns with the closest density, which are the safest substitutes.
What is tex and why does it matter?
Tex is the weight in grams of 1,000 m of yarn, the universal linear density unit defined in ISO 1144. It does not depend on how you wrap the yarn, so it is the most reliable way to match an unlabeled ball with a commercial yarn: 170 m per 100 g equals about 588 tex.