Yarn substitution

The three things that have to match

Weight category gets you in the right family. Wraps per inch confirms it — two yarns in the same category can still differ by a needle size. Gauge is the final test: swatch the substitute and compare it to the pattern's gauge before you commit to a sweater's worth of yarn.

Yardage is the part people get wrong. A skein is a quantity of length, not weight, and two 100 g skeins can differ by 50 yards. Always convert to total yards before comparing.

If the gauge does not match

Do not force it. Run the numbers through the gauge converter — it will tell you the new stitch count and the extra yarn the tighter fabric will need. If the difference is under 2%, knit as written.

What to check before you buy

Fibre first: a wool substitute for a cotton pattern will behave differently at the same gauge — heavier, warmer, less drape. Then weight, then wraps per inch, then yardage. Colour is last, because a dye-lot mismatch is annoying but a gauge mismatch wastes the whole project.

Buy the buffer skein. Ten percent covers swatching and joins on most projects, and a single extra skein of a discontinued yarn is cheap compared to ripping back a sleeve. If the substitute is a different dye lot from a second order, alternate skeins every few rows to blend.

Substituting inside the same weight category

The easy case: same category, similar wraps per inch, similar yardage. Match the fibre if the project depends on drape or warmth, then swatch. Two worsted yarns can still differ by a needle size, so the swatch is not optional — it is the only thing that tells you whether the substitute behaves like the original.

Watch the put-up. A 100 g skein of one yarn may hold 200 yards and another 260. Comparing price per skein is meaningless; compare price per yard, and compare total yards against what the pattern needs.

When the substitute is a different fibre

Fibre changes everything the gauge does not measure. Cotton is heavy and inelastic, so a cotton substitute for a wool sweater will grow in length and lose its bounce. Alpaca drapes and stretches; acrylic is light and pills; linen softens with washing and shrinks on the first one. If the pattern relies on blocking to shape lace, a synthetic substitute will not hold the block.

For garments, stay within the same fibre family unless you are prepared to re-fit. For accessories — hats, cowls, blankets — fibre swaps are low risk and often the point.

How to test a substitute before committing

Buy one skein, knit a 15 cm swatch in the pattern stitch, wash and block it, then measure gauge and feel. Compare it against the pattern's gauge and against the fabric you want. If it passes, buy the rest from the same dye lot in one order — matching a dye lot later is the most common way a substitution goes wrong after the maths was right.

Keep the swatch. When the project is finished and washed, measure it again: the difference between the blocked swatch and the worn garment tells you how that fibre behaves over time, which is the one thing no calculator can predict.

Questions people ask