Yarn substitution calculator

When you use a different yarn from the pattern, buy by length (yards or metres), not by weight. Two yarns in the same weight category can differ a lot in how much length you get per gram, and every stitch uses a length of yarn, not a weight. This calculator works out the balls to buy and shows how close the two yarns are.

How to use the calculator

1. Enter how many balls the pattern calls for, and their length and weight from the label.
2. Enter your yarn's length and weight per ball.
3. Check the length-per-gram comparison and any warning before you buy.

How the calculator works

First it finds the total length the pattern uses, then how many of your balls give that length:

Length needed = pattern balls × length per pattern ball

Your balls = length needed ÷ length per ball of your yarn, rounded up

If you enter both ball weights, it also compares the length per gram of the two yarns, often called grist:

Length per gram = ball length ÷ ball weight

A difference within 10% is a close match. Between 10% and 25%, swatch first and expect the piece to be noticeably heavier or lighter. Above 25%, the yarns usually differ in thickness or fibre, and the calculator warns you.

Reading your results

• Balls of your yarn: rounded up, with the exact figure underneath.
• Length needed: in yards and metres.
• Length per gram: the percentage difference between the yarns. Negative means your yarn is heavier for its length, so the finished piece will weigh more.
• Warning: shows how short you would be if you matched by weight instead.

Worked example

A sweater uses 6 balls of a yarn with 210 yd per 100 g, which is 1,260 yd. Your cotton blend has 165 yd per 100 g.

• Balls: 1,260 ÷ 165 = 7.64, so buy 8.
• Length per gram: 1.65 against 2.1 yd/g, 21.4% less. The sweater will weigh about 764 g instead of 600 g.
• Buying 6 balls (600 g) to match the weight would give only 990 yd: 270 yd short.

Accuracy and limits

• The arithmetic is exact and covered by our automated tests. It assumes you can match the pattern's gauge with your yarn.
• No extra is added; the result is the same length as the pattern, rounded up to whole balls. Add a ball if you want spare.
• If the pattern's ball count was already rounded up, you can enter its exact yardage instead: 1 ball of the pattern's total length.
• If you cannot match gauge without stiff or floppy fabric, recalculate the counts with the pattern gauge converter.

Common mistakes

1. Swapping balls one for one. Many yarns come in 50 g balls, others in 100 g or larger. Enter each yarn's own ball length and weight.
2. Ignoring how the fibre hangs. Even at the same gauge and length, a heavy, inelastic cotton or bamboo drapes differently from springy wool. Wash your swatch and check it with the blocking calculator.

Tips for more accurate results

• Swatch with the new yarn and compare it with the pattern in the gauge difference calculator.
• To replace one yarn with two thinner strands held together, find their combined length per gram with the yarn held together calculator.

Frequently asked questions