Yarn Substitution Calculator
Substituting by weight matches grams and nothing else. On a 1000 yard pattern, the same weight of a denser yarn leaves you 150 yards short.
The yarn the pattern names
The yarn you want to use
Side by side
Buy by length, not by weight
A ball band gives you both numbers, and only one of them is the thing you are using up. A pattern is a length of yarn worked into stitches; the grams are just what that length happens to weigh in that fibre. Substitute a denser fibre and the same weight buys you less yarn.
On a pattern needing 1000 yards of a wool at 220yd per 100g:
| Substitute | Yards per gram | Balls by length | Balls by weight | You end up with |
|---|---|---|---|---|
| Wool, 100g / 220yd | 2.20 | 5 | 5 | 1100 yd (+100) |
| Wool, 100g / 250yd | 2.50 | 4 | 5 | 1250 yd (+250) |
| Cotton, 100g / 170yd | 1.70 | 6 | 5 | 850 yd (-150) |
| Cotton, 50g / 85yd | 1.70 | 12 | 10 | 850 yd (-150) |
| Alpaca, 50g / 125yd | 2.50 | 8 | 10 | 1250 yd (+250) |
The cotton is the one that hurts. Matching the original's weight — 455 grams, so five 100g balls — leaves you 150 yards short, which on a sweater is somewhere around a sleeve. The alpaca goes the other way and costs you 250 yards you did not need to buy. Only the length column is ever right.
Wraps per inch: thickness you can measure
Yardage tells you how much yarn there is, not how thick it is — and a substitute has to match both or the fabric changes. Weight categories are unreliable for this: "worsted" covers a range, and mills disagree about its edges. Wraps per inch measures the yarn in front of you. Wind it round a ruler for an inch, without stretching and without overlapping, and count.
| Wraps per inch | Weight | Typically used for |
|---|---|---|
| 6 or fewer | Super bulky | Arm knitting and very fast blankets. |
| 7–8 | Bulky / chunky | Hats and outerwear. |
| 9–11 | Worsted / aran | The commonest garment weight. |
| 12–13 | DK | Light garments and baby knits. |
| 14–15 | Sport | Fine garments and baby knits. |
| 16–18 | Fingering / sock | Socks, shawls, fine colourwork. |
| 19+ | Lace | Lace and cobweb weight. |
More wraps means thinner yarn, which is backwards from every other number on a ball band and catches people out constantly. Lace is thirty-odd wraps to the inch; super bulky is six.
What the arithmetic cannot tell you
- Swatch anyway. Matching yardage and wraps per inch gets you close; only a swatch tells you whether you hit the pattern's gauge, and gauge is what decides the size.
- Fibre changes drape more than thickness does. Cotton and linen have almost no memory, so a cabled wool jumper substituted in cotton grows, sags and does not spring back. Alpaca drapes and stretches; superwash wool grows when wet.
- Plies change stitch definition. A round, tightly plied yarn shows cables and texture crisply. A singles or a loosely spun yarn blurs them, at identical yardage and identical gauge.
- Buy it all at once, in one dye lot. Running short is the failure that cannot be fixed later, because the shop will not have your lot. This is why the estimate is worth doing before you get to the till rather than after.
How to use
- Enter the yards and grams from both ball bands.
- Buy by the length column. The weight column is the trap.
- Check thickness with wraps per inch, not the weight name.
- Swatch before casting on, and buy the whole lot at once.
Frequently asked questions
How do I substitute one yarn for another?
Match the total LENGTH the pattern calls for, not the total weight. Work out how many yards the pattern needs, then divide by the yards per ball of your substitute and round up. Matching the original weight in grams is the common mistake and it is only correct when the two yarns happen to have the same yards per gram.
Why does buying the same weight not work?
Because yarns differ in how much length you get per gram. Cotton is denser than wool, alpaca is lighter. A pattern needing 1000 yards of a wool at 2.20 yards per gram works out to 455 grams — but 455 grams of a cotton at 1.70 yards per gram is only 850 yards. You would be 150 yards short, which on a sweater is roughly a sleeve.
What if the substitute is lighter than the original?
Then you overbuy rather than run short, which is the better failure but still costs money. The same 1000 yard pattern substituted with an alpaca at 2.50 yards per gram gives you 1250 yards if you match the weight — 250 yards more than you needed. Buying by length gets it right in both directions.
What are wraps per inch?
A way of measuring how thick a yarn actually is. Wind it round a ruler for one inch, without stretching it and without letting the wraps overlap, then count them. It compares two yarns directly, which a weight category cannot: worsted covers a range and manufacturers disagree about where its edges are.
Does more wraps mean thicker yarn?
No, the opposite, and this catches people out constantly because it is backwards from every other number on a ball band. More wraps fit in an inch when the yarn is thinner. Lace weight is around thirty wraps per inch; super bulky is about six.
Do I still need to swatch?
Yes. Matching yardage and wraps per inch gets you into the right territory, but only a swatch tells you whether you are hitting the pattern gauge — and gauge is what decides the finished size. A yarn can match on paper and knit at a different tension in your hands.
Can I substitute any fibre for any other?
You can, but the fabric will behave differently even at identical gauge. Cotton and linen have almost no memory, so a cabled wool jumper made in cotton grows and sags without springing back. Alpaca drapes heavily and stretches. Superwash wool grows when wet. The arithmetic is the easy part.
Does the ply matter?
For anything textured, a great deal. A round, tightly plied yarn shows cables and stitch patterns crisply. A singles or a loosely spun yarn blurs them, at exactly the same yardage and the same gauge. If the pattern is about its texture, match the construction as well as the thickness.
How much extra should I buy?
Round up to whole balls and consider one more beyond that, particularly for a first attempt or if you might lengthen anything. Running short is the failure that cannot be repaired later, because the shop will not have your dye lot when you go back.
What is a dye lot and why does it matter?
It identifies the batch a yarn was dyed in. Two balls of the same colour from different lots can differ enough to show as a visible line across a finished garment, especially in a solid colour. Buy the whole quantity in one lot, and if you must mix, alternate rows between the two so the change is gradual.
How do I work out the yardage if the pattern only gives balls?
Multiply the number of balls by the yards per ball of the yarn the pattern names — that figure is usually printed in the materials list. If it is not, the yarn name will find it. Do this before anything else, because the ball count on its own is meaningless once you change yarn.
Does this send anything anywhere?
No. Everything is calculated in your browser and nothing is stored or uploaded.
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