1. Match the gauge
The pattern's gauge — stitches and rows over four inches — is the only thing that must match. Start with yarns of the same weight class as the original; the weight name gets you into the right neighbourhood, and the ball band's suggested gauge narrows it. Then swatch: cast on enough for six inches, knit in the pattern's stitch, wash and dry the swatch the way you will treat the garment, and measure the middle four inches. If the stitch count is right and the rows are off, that is usually fine; if the stitches are off, change needle size and swatch again, and if that changes the fabric into something you do not like, choose a different yarn.
Every yarn page on this site lists substitutes we stock at the same weight and a similar fibre, which is a shortcut to the candidates.
2. Match yards per gram
Two worsted yarns can differ by a fifth in how much length a gram holds, and that is what decides whether the fabric is dense or airy at the same gauge. Divide the yardage on the band by the grams: a worsted merino at 210 yards to 100 g is 2.1 yards per gram; a worsted wool-mohair at 190 yards to 113 g is 1.7. The closer the substitute's number to the original's, the closer the fabric.
3. Match the fibre, or know what changes
Fibre decides drape, warmth and how the garment ages. Wool for wool is safe. Alpaca for wool is warmer and heavier and grows at the hem — good for a scarf, risky for a fitted sweater. Cotton for wool has no bounce: ribbing will not spring back and a heavy garment lengthens. Silk blends drape more; mohair adds halo and hides stitch detail. Superwash for non-superwash drapes more and holds shape less, and cannot be felted. A change of fibre is fine as long as the pattern does not depend on the property you are giving up — a cabled cotton sweater looks flat, a lace shawl in a springy wool will not open up.
4. Match the construction
Single-ply yarns show every stitch and bloom; tightly plied yarns give crisp cables and last longer. Self-striping and long-gradient yarns need a piece with an even row length to stripe cleanly — a hat or a scarf, not a raglan yoke. Tweed and heathers hide stitch detail slightly; solids show it.
5. Recalculate the skeins
Multiply the number of skeins the pattern calls for by the original yarn's yards per skein to get the total yardage. Divide that by the substitute's yards per skein, round up, add one. Never count skeins by skeins or grams by grams — a pattern that wants ten 50 g balls of one yarn does not want ten of another.
When in doubt
Tell us the pattern and the yarn you are thinking of. We do this at the counter every day.