Substituting Yarn Weights: A Complete Guide

Yarn substitution comes up constantly — the pattern calls for a yarn that's discontinued, too expensive, not in the color you want, or simply not available where you shop. Here's how to do it without guessing.

Step 1: Find the weight category

Check the pattern's yarn label or weight name, and find its category on ouryarn weight conversion calculator. Look for a substitute yarn in the same category first — it's the safest choice, since gauge, needle size, and hook size will already be close.

Step 2: Match gauge, not just the category name

Two yarns in the same weight category can still knit up at slightly different gauges depending on fiber and spin. Always knit or crochet a swatch with your substitute yarn at the needle or hook size the pattern suggests, then measure it against the pattern's stated gauge. Getting more stitches per inch than the pattern wants means your fabric is tighter than intended — go up a needle or hook size. Getting fewer means it's looser — go down a size. Keep adjusting and re-swatching until your gauge matches.

Step 3: Work out how much yarn to buy

This is the step people skip, and it's the one that causes running out mid-project. Find "yards per 100g" (or per skein) on both yarn labels — your original planned yarn and your substitute. Then:

1. Convert the pattern's required yardage to a weight, using the original yarn's yards-per-100g: divide the required yardage by that number, then multiply by 100 to get grams needed.
2. Convert that weight back to yardage using the substitute yarn's yards-per-100g: divide the grams by 100, then multiply by the substitute's yards-per-100g.

For example: a pattern needs 800 yards of a yarn rated at 220 yards per 100g. That's about 364g of fiber. If your substitute yarn is rated at 200 yards per 100g, you'd need roughly 727 yards of it to get that same 364g. Thinner substitute yarns (higher yards-per-100g) need more yardage for the same weight; thicker ones need less. Our yardage calculator does this math for you — just enter the three numbers.

This method assumes similar fiber density between the two yarns, which holds well for most substitutions within the same broad fiber family (wool-for-wool, cotton-for-cotton). It's an approximation, not a guarantee — buy one extra skein if you're working with a scarce or discontinued yarn and can't easily get more mid-project.

Step 4: Consider crossing categories

Sometimes there's no good match in the same category, and you need to substitute a different weight entirely — a lighter yarn held double, for instance, to approximate a heavier one. This works, but treat the gauge-matching step as non-negotiable: swatch with your actual substitution method (single strand, double strand, or held together with another yarn) before committing yardage or needle/hook size decisions.

Fiber content changes the outcome too

Gauge and yardage math get you a technically correct substitution, but fiber content decides whether the finished piece actually behaves the way the pattern intended. Wool has stretch and memory that cotton doesn't; cotton drapes heavier and doesn't bounce back the way wool does; acrylic tends to hold less warmth than wool at the same thickness. A lace shawl pattern written for a fine wool-silk blend will drape differently in a 100% cotton substitute, even at identical gauge. If drape and structure matter to the finished piece — garments especially — try to substitute within the same broad fiber family, or expect the result to differ from the sample photos.

Common substitution mistakes

• Matching the weight category name but skipping the gauge swatch entirely.
• Buying yardage based on the original yarn's requirement instead of recalculating for the substitute's yards-per-100g.
• Substituting across very different fiber types for a structured garment (like a fitted sweater) where drape and stretch recovery matter.
• Assuming "same brand, different colorway" always means identical gauge — dye processes can very slightly affect a fiber's hand and thickness.

When not to substitute

Some patterns are written specifically around one yarn's unusual properties — a self-striping colorway timed to stitch counts, a boucle or eyelash texture the stitch pattern is designed to showcase, or a yarn with unusual elasticity the fit depends on. In these cases, a same-weight substitute can technically knit up at the right gauge and still produce a noticeably different result. Read the pattern notes for any mention of why that specific yarn was chosen before assuming a straightforward swap will work.

Ready to substitute? Start with theconverter and chart to find your category, then use the calculator on that page for the yardage math.