Yarn Substitution Calculator
Substituting yarn asks two questions that do not have the same answer. How much do I need? is arithmetic on length and this calculator settles it. Will it make the right fabric? is a judgement, and no calculator settles that — so the two are reported separately, and you will not find a “great substitute” score anywhere on this page.
Two questions, two answers
Most substitution advice collapses into one number, and that is where it goes wrong. Whether you have enough yarn and whether you have the right yarn are independent, and a substitute can pass one while failing the other badly.
That is why the result above is split into two panels, and why no combined score appears. A single verdict would have to average an arithmetic certainty against a judgement, and the average would be worth less than either.
Quantity: convert to length, never skein for skein
Skein counts are not comparable between yarns, because skeins are not wound to a common length. The only transferable quantity is total length.
The calculator rounds up, because part of a skein is not purchasable, and reports what you will actually receive alongside what you needed. It also shows what a naive skein-for-skein swap would have given you, which is usually the more persuasive number.
Reading the two ball bands
Every figure a substitution needs is printed on the band, and each one does a different job.
Why the weight category is a shortlist, not a guarantee
The Craft Yarn Council weight system classifies yarn by thickness. It does not certify that two yarns in a category are interchangeable, and no standard does. Ply, twist, fibre and yards per 100 g all vary inside a single category, and each of them changes how the fabric behaves.
So a matching category earns a yarn a place on the shortlist. What settles it is a blocked swatch in the actual stitch pattern, compared against the pattern's stated gauge. The yarn weight converter covers the categories themselves; the yarn substitution guide covers the wider decision.
What each compatibility check can and cannot tell you
| Check | What it measures | What it cannot tell you |
|---|---|---|
| Gauge against the pattern | Whether your fabric hits the stitch density the pattern assumes | Whether the fabric feels right at that density, or how it moves after blocking |
| Weight category | Rough thickness, on the published 0–7 scale | Anything about ply, twist or drape — two yarns in a category can behave very differently |
| Fibre | Whether the material family has changed | How much it matters for your project; a cotton scarf and a cotton sweater fail differently |
| Yards per 100 g | How the yarn is built — a density and construction proxy | Which of spinning, fibre density or plying accounts for the difference |
These thresholds — 3% and 10% on gauge, 8% and 20% on yards per 100 g — are CrochetCalc's own bands for turning a measurement into readable advice. No standards body publishes substitution tolerances, and none is implied. The weight categories themselves are the Craft Yarn Council's; the judgement about what they do and do not guarantee is ours, and is stated so you can disagree with it.
Three worked examples
A straightforward swap. The pattern wants 8 skeins at 220 yd, so 1,760 yd. The substitute is wound at 170 yd, giving 10.35 skeins, rounded to 11. With a 10% buffer the requirement rises to 1,936 yd and 12 skeins. Buying 8, matching the pattern's count, would have left you 400 yd short.
Metric bands, imperial substitute. The pattern lists 6 skeins at 200 m, which is 1,200 m or about 1,312 yd. A substitute at 220 yd per skein needs 6 skeins — the same count by coincidence, not by rule.
Enough yarn, wrong yarn. A worsted wool pattern needing 1,760 yd, substituted with a fingering cotton at 800 yd per skein. Quantity says 3 skeins. Compatibility reports gauge off by 75%, three categories apart, and yards per 100 g off by 264%. The arithmetic is right and the substitution is not.
Where substitutions go wrong
Buying by skein count. The single most common error, and the most expensive when the dye lot has gone by the time you notice.
Treating the weight category as permission. It is a filter, not a verdict.
Skipping the swatch because the band gauges match. Band gauge is the manufacturer's, worked in their stitch at their tension. Yours will differ — the gauge swatch calculator compares them properly.
Ignoring fibre on a garment. Cotton has little memory and grows under its own weight; wool springs back. On a scarf that difference is cosmetic, on a fitted cardigan it is structural.
Frequently asked questions
How many skeins of substitute yarn do I need?
Convert to total length, never skein for skein. A pattern calling for 8 skeins at 220 yards needs 1,760 yards; a substitute wound at 170 yards per skein therefore needs 1,760 ÷ 170 = 10.35, rounded up to 11 skeins. Buying 8 substitute skeins because the pattern said 8 would leave you 400 yards short.
Can I substitute any yarn of the same weight?
Sharing a CYC weight category makes two yarns candidates, not equivalents. The category describes thickness and nothing else. Two worsted yarns can differ in fibre, ply, twist and yards per 100 g, and each of those changes the fabric. Match the category to build a shortlist, then check gauge and swatch.
What is the difference between quantity and compatibility?
Quantity is arithmetic on length and it is reliable. Compatibility is a judgement about whether the substitute will make the fabric the pattern intends, and no calculator can settle it. This page reports them in separate panels because a passing quantity result says nothing about the second question.
Do I need to swatch if the substitute is the same weight category?
Yes. Category is a shortlist, gauge is the test. Two yarns in the same category routinely produce different stitch counts over 4 inches, and drape after blocking varies further still.
Why does length per 100 g matter?
It is a proxy for how the yarn is built. Two worsted yarns at 220 and 170 yards per 100 g are differently spun or differently dense, which shows up as weight, drape and stitch definition even when the gauge happens to match.
How much extra should I buy when substituting?
More than for the original, because a substitution adds uncertainty on top of the usual tension drift and dye-lot risk. This calculator takes a buffer as an input rather than assuming one, since the right margin depends on project size and how easily you could buy more later.
Sources & further reading
- Yarn substitution guide — the full five-step method this calculator sits inside.
- How to read yarn labels — finding length, weight, gauge and fibre on the band.
- Yarn fibre types — how wool, cotton, acrylic and blends behave differently.
Quantity here is arithmetic on the lengths you enter, converted at 1 yd = 0.9144 m. Compatibility is a set of independent checks against thresholds this site chose; they are advisory and are shown separately from the arithmetic for that reason. Nothing on this page has been tested against physical yarn, and no such claim is made.
Related tools
- Yarn weight converter — CYC categories, WPI and what the numbers on the band mean.
- Yarn yardage calculator — total length from a swatch when the pattern does not state one.
- Gauge swatch calculator — compare your swatch against the pattern before committing.