Patterns are written for one specific yarn, but they almost never have to be made with it. Knitters and crocheters substitute yarns all the time: to use stash, to change fiber for an allergy or the washing machine, or because the original yarn was discontinued years ago. The two classic mistakes are buying the same number of skeins as the pattern and choosing a yarn that is only nominally the same weight. Both lead to running short or to a garment that comes out the wrong size.
Step 1: find the total length the pattern needs
Patterns list yarn as a number of skeins of a particular yarn, with that yarn's length and weight: "9 skeins of Brand X Worsted, 100 g / 210 yd each". Multiply the two: 9 × 210 = 1,890 yards. This total length is what you need, whatever yarn you choose. If the pattern lists sizes, use the number for your size.
Skeins of different yarns contain very different lengths. A 100 g worsted might be 170 yards in wool, 210 in a wool blend, or 364 in a 198 g "super saver" acrylic. Buying "9 skeins" of a 170-yard yarn would leave you more than 300 yards short.
Step 2: match the thickness
Start with the same weight category — the number from 0 to 7 on the label — and then compare yards per 100 grams, which is a more precise measure of thickness:
yards per 100 g = label yards ÷ label grams × 100
If the original yarn is 210 yd / 100 g and the substitute is 220 yd / 100 g, they differ by under 5%: a good match. Within about 10% the yarns will usually knit to the same gauge with the same needles. Between 10% and 20%, swatch carefully; you may need a different needle or hook size, and the drape will differ. Beyond 20% you are effectively in a different weight category, and you should look for a different yarn or recalculate the pattern.
Remember that fiber shifts this figure. Cotton is dense, so a cotton worsted may be only 170–180 yd / 100 g but knit to the same gauge as a 210-yard wool. When fibers differ, trust the gauge on the label (stitches per 4 inches) and your swatch more than the yards-per-gram figure.
Step 3: compare fiber and construction
- Elasticity. Wool springs back; cotton, linen, bamboo and silk do not. A ribbed hat in cotton will not grip like wool; a cotton sweater will grow lengthwise.
- Weight of the fabric. A dense cotton or superwash sweater weighs more and can stretch under its own weight.
- Halo and texture. Mohair, brushed alpaca and bouclé hide stitch patterns; smooth, tightly plied yarns show cables and texture best.
- Care. Baby items and blankets usually need to be machine washable.
Step 4: calculate how much of the new yarn to buy
Divide the total length by the length of one skein of the new yarn, and always round up:
new skeins = ⌈ pattern total yards ÷ new yarn yards per skein ⌉
Example: the pattern needs 1,890 yards. Your substitute has 220 yd per 100 g skein, so 1,890 ÷ 220 = 8.6 → 9 skeins, with about 90 yards left over. If the substitute were a 364-yard skein, you would need 1,890 ÷ 364 = 5.2 → 6 skeins.
If the substitute is a little thinner (more yards per gram) and you adjust needle size to match gauge, the length you need stays about the same. If you keep the needles and the yarn is thinner, your fabric will be denser and smaller — so you would need to recalculate the pattern anyway.
The yarn substitution calculator does these steps: enter both labels and the number of skeins (or the total length) and it shows yards per 100 g for both yarns, their difference, the likely weight category and how many skeins of the new yarn to buy.
Step 5: swatch and adjust
Knit or crochet a swatch with the new yarn in the pattern stitch, wash and block it, and compare gauge with the pattern. Change needle or hook size until the stitch count matches. Then weigh the swatch: the swatch calculator will tell you whether your total length still holds for this yarn and your tension.
Substituting with several strands
Some patterns use two strands held together — for example a fingering wool with a lace mohair. You can substitute each strand separately (the mohair with another lace mohair) or replace the combined strand with a single thicker yarn of the same gauge. If you do the latter, you need the length of only one strand, not two, but the thickness must match the combined pair: two DK strands knit roughly like one bulky yarn.
Substitution checklist
- Total length = skeins × yards per skein from the pattern.
- Same weight category; yards per 100 g within 10%.
- Similar fiber behaviour (elasticity, weight, care).
- Buy ⌈total ÷ new skein length⌉, plus one skein if the project is large.
- All skeins from the same dye lot — see the dye lot guide.
- Swatch, block, measure, and adjust needle size before you start.