Wraps Per Inch (WPI) Calculator
Wraps per inch (WPI) measures how many adjacent wraps of yarn fit within one inch. Enter what you counted and this calculator reports every Craft Yarn Council weight range your reading falls inside — which is often more than one, because the published ranges share values. It identifies yarn thickness; it does not prove that one yarn will substitute for another.
What Does Your WPI Measurement Mean?
A WPI reading places your yarn on a thickness scale. It does not name the yarn. The Craft Yarn Council publishes eight weight categories, numbered 0 to 7, each with a WPI range — and those ranges overlap, so a single reading commonly matches several categories at once.
| WPI range | Reference category | What this means | Where it overlaps |
|---|---|---|---|
30–40+ | 0 · Lace | The finest published range, and open-ended upwards. | Only Super Fine overlaps it, at exactly 30. |
14–30 | 1 · Super Fine | Sock and fingering yarns sit here. | Shares 14–15 with Light and 14–18 with Fine. |
12–18 | 2 · Fine | Sport weight. | Shares 12–15 with Light and 12 with Medium. |
11–15 | 3 · Light | DK and light worsted. | Overlaps Medium, Fine and Super Fine — the most crowded part of the scale. |
9–12 | 4 · Medium | Worsted and aran. | Shares 9 with Bulky and 11–12 with Light. |
6–9 | 5 · Bulky | Chunky yarns. | Shares 6 with Super Bulky and 9 with Medium. |
5–6 | 6 · Super Bulky | The narrowest published range. | Shares 6 with Bulky. |
1–4 | 7 · Jumbo | Roving and arm-knitting weights. | No overlap, but nothing is published between 4 and 5. |
Ranges are the Craft Yarn Council's, reproduced as published. The overlap column is this page's own comparison of them.
How the WPI Calculator Works
The arithmetic is deliberately plain, because anything more would be invented precision.
One reading, measured over one inch: your WPI is the wrap count. Measured over a longer span: WPI = wraps ÷ inches, so 25 wraps over 2 inches is 12.5 WPI. Several readings: the calculator averages them — the sum divided by the number of readings — and also reports the spread between your lowest and highest, because that spread tells you how repeatable your wrapping was.
The result is then compared against the published ranges. Three outcomes are possible, and the calculator states which one you got: the reading falls inside exactly one range, inside several, or inside none. It never picks a nearest category. A reading of 4.5 WPI sits above Jumbo (1–4) and below Super Bulky (5–6), and since the Council publishes nothing in between, the honest answer is to say so.
How to Measure Wraps Per Inch Correctly
Wrap the yarn around something with a consistent circumference — the Craft Yarn Council names a pencil, and a dowel or a purpose-made WPI tool works too. The strands should lie side by side and touch, with no gaps between them and none riding over another. Keep the yarn snug — not stretched. Count the wraps that fall inside one marked inch.
Two habits improve the reading more than anything else. Measure in a few places along the strand, as you would when checking a gauge swatch, and average them — that is what the multiple-readings input above is for. Practise on a yarn whose weight you already know, so you can calibrate your own wrapping tension before trusting it on a mystery yarn. Both are the Craft Yarn Council's own advice. The full method, including what to do with haloed and elastic yarns, is in how to measure wraps per inch.
Why WPI Ranges Can Overlap
This is the part most WPI tools quietly skip, and it is the single most useful thing to understand about the measurement.
Counting whole-number readings from 1 to 45, eleven of them match more than one published category. The crowded middle of the scale is worst: 12, 14 and 15 WPI each match three categories. Only the extremes — the Jumbo end and the far Lace end — give a single unambiguous answer.
The overlap is not sloppiness. Yarn thickness is continuous, categories are discrete, and the Council chose to publish honest ranges with shared edges rather than invent hard boundaries that no yarn respects. A tool that reports one category from one reading is hiding that, not solving it.
WPI vs Yarn Weight vs Crochet Gauge
Three related measurements, three different jobs, and conflating them causes most of the confusion around mystery yarn.
WPI measures the strand. It describes the yarn as it sits in your hand, unworked. Yarn weight category classifies the yarn into a published band, and the yarn weight conversion guide covers what each category means. Gauge measures the fabric — stitches and rows per four inches, once the yarn has been worked with a particular hook by particular hands.
Only the third one predicts what your project will do, because only the third one involves your hook and your tension. WPI narrows the field; gauge settles it. The gauge swatch calculator is the step after this one.
When WPI Is Useful
A yarn with no ball band. This is the case WPI exists for. A reading takes you from nothing to a shortlist of two or three plausible categories, which is enough to pick a starting hook and swatch.
Checking a suspicious label. Hand-dyed and small-batch yarns are sometimes labelled optimistically. A WPI reading that disagrees sharply with the stated category is a reason to swatch before trusting the pattern's yardage.
Comparing two yarns you already own. Measuring both under the same wrapping tension is more informative than comparing either against the table, because your consistent error cancels out between them.
When WPI Is Not Enough
It cannot confirm a substitution. Two yarns at 12 WPI can differ in fibre, twist, drape and yards per 100 g. Thickness is one variable among several — the yarn substitution guide covers the rest.
It cannot predict yardage. Length per unit mass depends on fibre density as well as thickness, so two yarns of identical WPI can differ substantially in yards per 100 g.
It struggles with unusual construction. A chainette, a bouclé, a thick-and-thin single or anything with a halo does not have one thickness to measure. The reading will be repeatable and still misleading.
It says nothing about your gauge. Two crocheters measuring the same yarn at the same WPI will still produce different fabric, because gauge includes the hand holding the hook.
Common WPI Measurement Mistakes
Wrapping too tightly. The most common error by some distance. Tension compresses the strand, packs more wraps into the inch and reports a thinner yarn than you have. Snug means the strands touch, not that they squash.
Leaving gaps. The opposite error, and it reads thicker. Strands should touch along their whole length.
Taking one reading. A single measurement has no way of telling you how repeatable it was. Three readings and their spread do.
Counting the wraps rather than the inch. Count what falls inside the marked inch, not the wraps you made. If a partial wrap sits on the line, either include it consistently or wrap over a longer span and divide.
Measuring stretched yarn. Pulling from a ball under tension thins the strand. Let a length relax first.
Treating the result as the answer. WPI narrows the field. The swatch decides.
Frequently Asked Questions
What yarn weight is 12 WPI?
Twelve wraps per inch falls inside three published Craft Yarn Council ranges at once: Medium (9–12), Light (11–15) and Fine (12–18). That is not a fault in your measurement — the published ranges share values rather than meeting at boundaries. Treat 12 WPI as narrowing the field to three categories, then settle it with a gauge swatch.
How many wraps per inch is worsted yarn?
The Craft Yarn Council lists Medium (category 4), which includes worsted, at 9 to 12 wraps per inch. Bulky is 6 to 9 and Light is 11 to 15, so a reading of 9 or 12 is consistent with worsted and with a neighbouring category as well.
Is WPI an accurate way to identify yarn?
It is a useful clue rather than an identification. The Craft Yarn Council states plainly that “WPI is subjective, and results will vary depending on how tightly the yarn is wrapped.” Wrapping loosely leaves gaps and reads low; wrapping tightly compresses the strands and reads high. Take several readings in different places and compare the average against the ranges.
How do I measure wraps per inch?
Wrap the yarn around something with a consistent circumference — the Craft Yarn Council names a pencil — so the strands lie side by side, touching, with no gaps between them and none riding over another. Keep the yarn snug rather than stretched. Then lay a ruler alongside and count the wraps that fall inside one marked inch. If you wrap over two inches instead, divide the count by two.
Why does my WPI not match the yarn's label?
Fibre and construction change how a yarn behaves under wrapping, and so does your own wrapping tension. The label describes the yarn as sold; WPI measures the strand as it is in your hand today. Which constructions read high, which read low, and what to do about haloed or elastic yarns is covered in the measurement guide.
Can WPI tell me if a yarn will substitute for another?
No. WPI estimates thickness, and thickness alone does not settle substitution. Two yarns at the same WPI can differ in fibre, twist, drape and yards per 100 g. Use WPI to shortlist, then compare stated gauge and swatch both yarns.
What if my reading falls between two published ranges?
It can happen with a fractional reading: 4.5 wraps per inch sits above Jumbo (1–4) and below Super Bulky (5–6), so no published range contains it. This calculator reports that rather than rounding you into the nearer category, because the Council does not define the space between.
Sources & further reading
- Craft Yarn Council — How to Measure Wraps Per Inch (WPI) — the published ranges and the measuring method used on this page.
- How to measure wraps per inch — the full measurement method this calculator assumes you have followed.
- Yarn weight conversion guide — what each of the eight categories means once you have narrowed it down.
- How to read yarn labels — for the yarns that still have one.
The WPI ranges above are the Craft Yarn Council's, retrieved on 13 August 2026 and confirmed by a second, differently worded reading of the same page that agreed on every value checked. The Council publishes eight categories, 0 to 7. It states that “WPI is subjective, and results will vary depending on how tightly the yarn is wrapped”, which is why this calculator reports a spread and never a confidence figure — the Council publishes ranges, not distributions, so any percentage would be invented. The overlap analysis, the count of ambiguous readings and the gap between the Jumbo and Super Bulky ranges are CrochetCalc's own comparison of the published values. Wrapping-tension advice is the Council's; the fibre and construction notes are practitioner convention.
Related tools
- Gauge swatch calculator — the verification step after WPI: what your hook and hands actually produce.
- Yarn weight converter — once you have a category, convert yardage between weights.
- Yarn substitution calculator — quantity and compatibility when replacing one yarn with another.