Guides · Knitting & Crochet
How to substitute yarn in any pattern
The pattern names a yarn that is discontinued, out of budget, or only sold on another continent. Your stash, meanwhile, is full. Substitution is how those two facts meet safely: three matches to make in order, one piece of honest arithmetic, and a swatch that has the final word over all of it.
Match one: the weight, by the number
Every safe substitution starts with the Craft Yarn Council weight number, the 0 to 7 scale printed on the ball band. Same number first: a pattern written for CYC 4 medium gets another CYC 4 yarn. The number is the one part of the band that cannot mislead you, because the names around it travel badly: worsted means something in the US and nothing in the UK, DK the reverse, and aran sits on bands on both sides of the Atlantic meaning slightly different things each time. The full scale, with the gauges and tool sizes each weight expects, is laid out in yarn weights, explained.
Half a weight out is where trouble begins. A heavy DK substituted for a light worsted can look identical on the shelf and knit up a size small, because gauge follows the strand, not the label's confidence. If the number matches, you have a candidate. If it does not, no amount of clever maths downstream makes the swap safe for anything fitted. Put it back.
Match two: the fibre's behaviour
Weight says how thick the strand is. Fibre says how the finished fabric will behave on your body and in your wash, and two CYC 4 yarns can behave like different materials entirely:
| Fibre | Stretch and memory | Drape | The honest catch |
|---|---|---|---|
| Wool | Real spring: it stretches and returns, and holds the shape it was blocked to | Moderate, holds a structured shape | Can felt with heat and agitation; some people itch; costs the most |
| Cotton | Almost no stretch and no memory: what it stretches to, it stays | Heavy and fluid, hangs rather than springs | Garments grow downward under their own weight and do not grow back |
| Acrylic | Some spring, little true memory; keeps a shape adequately, not faithfully | Light, can be slippery or squeaky depending on quality | Does not block like wool, and steam can relax it permanently; quality varies enormously |
| Blends | Behave mostly like their biggest share, with manners from the rest | Usually between the parents | Read the percentages, not the headline fibre on the front of the band |
The practical rule: swap inside a behaviour family for anything fitted. Wool for wool, or wool for a wool-rich blend, and the pattern's shaping, ease and blocking instructions all still mean what they meant. Cotton for an acrylic dishcloth yarn changes almost nothing, because a dishcloth asks so little of its fibre. Cotton for the wool in a fitted sweater changes the garment itself, which the failure section below comes back to.
Never match a substitution by the weight of the ball. Two 100 g balls can hold completely different lengths, because length follows the thickness and the fibre, not the scale. Patterns ask for yardage for exactly this reason. Everything in the next section runs on yards and metres read off the band, and how to read a yarn label shows where those numbers hide.
Match three: the yardage maths
One formula runs every substitution, and it is the whole of it:
balls to buy = round up( total yardage the pattern needs ÷ yards on one new ball ) + 1 spare ball
Round up because shops do not sell fractions of a ball, and add the spare because dye lots do not wait, swatches eat yarn, and sleeves have a way of growing. Two worked examples, with the numbers carried all the way through:
Example one: the First Sweater in a different skein
First Sweater, No Tears is written in a CYC 4 worsted, and the size M (finished bust 42 in / 107 cm) needs 1,450 yd (about 1,326 m). The yarn our shelf plans around comes in 220 yd skeins, so the pattern's own shopping list runs: 1,450 ÷ 220 = 6.6, round up to 7 skeins (1,540 yd on the shelf, the pattern's figure plus its working margin).
Now swap in a different worsted, same CYC 4, a smooth wool that comes in smaller 197 yd skeins. The pattern's need has not changed, only the put-up: 1,450 ÷ 197 = 7.4, round up to 8 skeins. Check it: 8 × 197 = 1,576 yd, which covers the 1,450 with about 9 percent to spare, while 7 skeins would give 1,379 yd and run dry somewhere in the second sleeve. Add the spare ball and you buy 9 skeins (1,773 yd), all in one dye lot. Same sweater, same weight class, three different numbers on the receipt, and only the division changed.
Example two: the dishcloth cotton swap
The easy swap: same weight, same fibre family, different ball. The Dishcloth Course cloth uses about 75 yd of CYC 4 cotton, so a set of three needs 225 yd. The course plans around a 120 yd cotton ball: 225 ÷ 120 = 1.9, round up to 2 balls (240 yd).
Your shop stocks its cotton in 100 yd balls instead. 225 ÷ 100 = 2.25, round up to 3 balls (300 yd), and the spare ball rule says take 4 if you want the set to match exactly and never think about it again. Sanity check at the small end: one cloth is 75 yd, so a single 100 yd ball still makes one cloth with 25 yd left over, just like the bigger ball did. Cotton for cotton at the same weight is the kindest substitution there is, because nothing about the fabric's behaviour changes, only the shopping.
same number, same yardage, then let the swatch argue
The verdict: your swatch, not the label
Everything above earns a yarn the right to be swatched. Nothing above lets it skip the swatch. The gauge printed on a ball band is the maker's average across many knitters; your hands, your needles and this yarn will produce your gauge, and the two can differ by a stitch or more per 4 in, which across a sweater is a whole size. Knit or crochet a swatch bigger than the 4 in square you will measure, at least 5 to 6 in across, wash and block it the way the finished piece will be treated, then measure the middle of it.
The First Sweater's gauge is 18 stitches over 4 in: that is the number a substitute has to hit, in a fabric you actually like the feel of. A yarn that only reaches gauge when it is worked so tight it feels like cardboard, or so loose it shows every strand of stuffing and daylight, has failed the substitution no matter how perfect the arithmetic was. Change needle or hook size once, reswatch, and if it still misses, believe the swatch and pick another yarn. The full swatching method is in gauge, explained.
When the swatch is measured, the gauge calculator turns it into decisions: enter your stitches and rows over 4 in, and it works out your cast-on counts and whether the fabric will land on the pattern's finished size. Run the substituted yarn through it before the balls leave the shop, not after.
When substitution fails, said plainly
| The swap | Why it fails |
|---|---|
| A fitted sweater pattern, wool swapped for pure cotton | The pattern's fit leans on wool's memory. Cotton stretches under the garment's own weight and stays stretched: the sweater grows, the waist shaping sags, and no blocking session calls it back. |
| Lace in a smooth, halo-free yarn | Half of lace is the halo and the memory that opens and holds the patterning at blocking. A crisp, springless yarn gives you the holes without the bloom, and the shawl reads as netting. |
| Colourwork in two yarns of different character | Stranded work needs both yarns to stretch, block and wear together. A wool and an acrylic in one yoke pucker apart at the first wash and age at different speeds. |
| A drape-driven design in a springy yarn | Some patterns are the drape: a fluid shawl or a bias skirt in a bouncy wool comes out lively instead of liquid. The maths matched and the garment is still wrong. |
None of these are shopping failures; they are honesty failures, usually at the moment someone falls in love with a ball and goes looking for reasons. The correct move when a substitution fails is not a bigger swatch or a sterner needle. It is a different pattern, written for the yarn you actually have. Your stash is not short of yarn. It is short of the right introductions.