Wraps per inch (WPI) calculator

Found a ball with no label, or a yarn whose weight category you are not sure of? Wrap it round a ruler. This calculator turns the wraps you count into wraps per inch (WPI) and matches them to the Craft Yarn Council weight categories, with the usual hook range and single crochet gauge. If you know the ball's length and weight, it cross-checks the result.

How to use the calculator

1. Wrap the yarn snugly round a ruler, pencil or dowel, strands side by side without gaps, overlaps or stretching.
2. Count the wraps over 2 in (5 cm) and enter both numbers. Two inches is steadier than one.
3. If you know the ball's length and weight, enter them as a cross-check. For an unlabelled ball, wind off a length, measure it and weigh it.

How the calculator works

WPI = wraps ÷ inches covered

Length per 100 g = length ÷ weight × 100

Each is compared with the category ranges from the Craft Yarn Council. The ranges overlap at their edges, so when the WPI fits two categories, the length per 100 g usually decides. Since the wraps lie side by side, the yarn is roughly 25.4 ÷ WPI mm thick.

If thickness and weight point to different categories, the fibre is unusually dense or light: cotton and linen are heavy for their thickness, while brushed and blown yarns are light. Thickness decides stitch size, so swatch with the hook for the thickness category; the calculator warns when they disagree.

Reading your results

• Yarn weight: the matching category.
• Wraps per inch: with the approximate thickness of the yarn.
• Per 100 g: the length, in yards and metres, when you entered the label.
• Category table: other names for the category (including UK names), its WPI range, a typical single crochet gauge per 4 in and a starting hook range.

Worked example

You wrap a mystery yarn round a dowel and count 22 wraps over 2 in. A torn tag says 220 yd per 100 g.

• WPI: 22 ÷ 2 = 11, so the yarn is about 2.3 mm thick. 11 WPI is on the border between 3 Light (11 to 15) and 4 Medium (9 to 12).
• Label: 220 yd per 100 g is 201 m, which is in 3 Light (200 to 260 m).
• Result: 3 Light, also called DK or light worsted (UK DK), about 12 to 17 single crochet per 4 in, starting hook 4.5 to 5.5 mm.

Accuracy and limits

• This is a starting point, not a measurement of your gauge, and our automated tests check the arithmetic and the category lookup. Swatch to confirm.
• Wrapping tension changes the count. Wrap snugly, but do not pull.
• Thick-and-thin and textured yarns give uneven counts; count over a longer length.
• Our yarn weights guide, linked below, explains each category in more detail.

Common mistakes

1. Pulling the yarn tight as you wrap. Stretching squeezes out the loft and adds wraps, making a yarn look thinner than it is.
2. Pushing the wraps together. They should sit side by side as they lie naturally, not packed into a ridge.
3. Counting over 1 in. Small errors at the edges count for more; use 2 in or 5 cm.

Tips for more accurate results

• Once you know the category, weigh the ball and use the remaining yarn calculator with a typical length per 100 g to estimate what is left.
• If you are combining strands, the yarn held together calculator works out their combined WPI.

Frequently asked questions

Related guides and tools

• Yarn weight categories explained