How to Calculate How Many Balls of Yarn You Need: Convert Substitutes by Length
When you knit with a yarn other than the one in the pattern, comparing by weight may not tell you how many balls to buy. This article covers the formula for converting a substitute yarn by length (meters), worked examples, how much spare to allow, how to estimate a whole project's yarn from a swatch's weight, and what to watch for when buying, such as dye lots and textured stitches.
Convert substitute yarn by length, not weight
A yarn label gives the weight and length per ball, such as "40 g ball, about 80 m." When you use a different yarn from the pattern, converting by weight throws off the amount for yarns of different thickness or material. That's because what a project uses is largely determined by the length of yarn you knit.
| How to convert | Calculation | Result |
|---|---|---|
| By length (correct) | 80 m × 8 balls = 640 m; 640 ÷ 120 m = about 5.3 balls | 6 balls with 10% spare |
| By weight (off) | 40 g × 8 balls = 320 g; 320 ÷ 50 g = 6.4 balls | 7 balls (1 too many) |
In this example, the pattern yarn is 40 g, 80 m (2 m per gram), and the yarn you're using is 50 g, 120 m (2.4 m per gram). Your yarn is 1.2 times longer per gram, meaning thinner, so comparing by weight makes you overbuy. If your yarn is thicker (shorter per gram), converting by weight leaves you short.
The formula and worked examples
| Pattern yarn | Your yarn | Length needed | Balls calculated | With 10% spare |
|---|---|---|---|---|
| 40 g, 80 m × 8 balls | 50 g, 120 m | 640 m | About 5.3 | 6 |
| 40 g, 88 m × 12 balls | 50 g, 100 m | 1,056 m | About 10.6 | 12 |
| 40 g, 100 m × 10 balls | 40 g, 80 m | 1,000 m | 12.5 | 14 |
| 50 g, 200 m × 6 balls | 100 g, 220 m | 1,200 m | About 5.5 | 6 |
As in row 3, even at the same 40 g, a shorter (thicker) yarn takes more balls. In the tool's "Yarn balls" tab, enter the weight and length per ball of both yarns, the ball count and the spare percentage to see the length and balls needed, the difference from converting by weight, and a thickness comparison of the two yarns (length per gram).
Estimating a whole project from a swatch's weight
For a project with no pattern or in your own size, you can estimate the amount from a swatch's weight. Fabric weight is roughly proportional to area, so calculate: swatch weight ÷ swatch area × project area.
- 1
Weigh the swatch
If a 15 cm × 15 cm (225 cm²) swatch weighs 7 g, that's about 0.031 g per cm². A kitchen scale that reads to 0.1 g improves accuracy.
- 2
Work out the project's area
For a sweater, treat the front and back (50 cm wide × 62 cm long × 2) and sleeves (40 cm wide × 48 cm long × 2) as rectangles: 6,200 + 3,840 = 10,040 cm².
- 3
Convert to weight and balls
7 ÷ 225 × 10,040 = about 312 g. With 40 g balls, that's about 7.8 balls, or 9 with 10% spare.
| Project | Swatch | Area | Weight needed | 40 g balls (10% spare) |
|---|---|---|---|---|
| Sweater (2 body panels, 2 sleeves) | 7 g per 15 cm square | 10,040 cm² | About 312 g | 9 |
| Scarf (20 cm × 150 cm) | 4 g per 12 cm square | 3,000 cm² | About 83 g | 3 |
| Hat (52 cm around × 22 cm deep) | 4 g per 12 cm square | 1,144 cm² | About 32 g | 1 |
Treating sleeves and body as rectangles slightly overestimates by the armhole and neckline curves, and that difference serves as natural spare. Ribbing and textured sections have different densities, so a separate swatch for each gives a more accurate weight.
Yarn amounts when you change size
When making a pattern larger or smaller, adjust the balls by the ratio of areas, not dimensions. Both width and length change, so dimensions 10% larger mean an area of 1.1 × 1.1 = ×1.21, or about 20% more.
| Size | Body (1 panel) | Sleeve (1) | Total area | Weight needed | 40 g balls (10% spare) |
|---|---|---|---|---|---|
| 10% smaller | 45 cm × 55.8 cm | 36 cm × 43.2 cm | About 8,132 cm² | About 253 g | 7 |
| Original size | 50 cm × 62 cm | 40 cm × 48 cm | 10,040 cm² | About 312 g | 9 |
| 10% larger | 55 cm × 68.2 cm | 44 cm × 52.8 cm | About 12,148 cm² | About 378 g | 11 |
This table is based on a swatch of 7 g per 15 cm square, with 2 body panels and 2 sleeves calculated as rectangles. A 10% difference in dimensions changes the yarn needed by 2 balls either way. Thinking "10% bigger, so 10% more balls" easily leaves you short. If you only lengthen the piece, the area grows just by the ratio of the length.
What to watch for when buying yarn
- Match dye lots. Even with the same color number, different lots vary slightly in shade, which can show as stripe-like bands in the fabric
- About 10% spare is the guide. Cables and textured stitches use more yarn at the same dimensions, so allowing about 20% is safer
- For textured projects, calculating from the weight of a swatch knit in the pattern avoids overlooking the difference from stockinette
- The same lot may be unavailable if you run short, so buy a little extra and use leftovers for small items or repairs
A guide to matching yarn thickness with needles is in Knitting Needle and Crochet Hook Sizes in mm. Use it when choosing a yarn of a different thickness.
FAQ
- How many balls should I buy when knitting with a different yarn from the pattern?
- Convert by length. A pattern with 8 balls of 80 m yarn needs 640 m. If your yarn is 120 m per ball, that's 640 ÷ 120 = about 5.3 balls, or 6 with 10% spare.
- Can't I convert by weight?
- Weight is roughly right for yarns of the same thickness and material, but not for different yarns, because the length per gram differs. Switching to a thinner yarn makes you overbuy, and a thicker one leaves you short.
- How many grams of yarn does a sweater need?
- It depends on yarn thickness and size, but about 300–500 g is a guide for an adult sweater in medium-weight yarn. Calculating from a swatch's weight, a fabric of 7 g per 15 cm square gives about 312 g for a sweater of about 10,000 cm² of body and sleeves.
- How much spare should I buy?
- About 10% is the guide. For projects heavy in cables or textured stitches, or ones where you'll adjust the size, about 20% is safer. The same lot may not be available later, so buying a little extra is recommended.
- What happens if yarn lots differ?
- Even with the same color number, yarn dyed at different times can vary slightly in shade. Changing lots partway can leave a visible band at the join, so buy all the yarn for a project from the same lot.
Tools in this article
Related articles
How to Measure Knitting Gauge: Recalculating Stitches and Rows When It Differs
When you've followed the pattern yet your sweater comes out too big or too small, gauge is usually the reason. This article covers what gauge means (stitches and rows in a 10 cm square), how big a swatch to make and how to measure it properly, whether to change needles or stitch counts when it doesn't match, the formulas for adapting a pattern's stitches and rows to your gauge, and how to round to a pattern's repeat, such as 2×2 rib.
Read article →Rice-to-Water Ratio Chart | How Much Water for 1 to 5 Cups (Gō) of Rice in a Rice Cooker, Clay Pot, Saucepan or Pressure Cooker
“How many millilitres of water for 3 cups of rice?” “Does a clay pot need more than a rice cooker?” “Wash-free rice needs a little more and new-crop rice a little less, but how much?” When the lines inside the rice cooker bowl are worn off, when you cook in a pot, or when you switch to a different kind of rice, the right amount of water suddenly becomes hard to judge. For white rice, the guideline is 1.1 times the volume of the washed, drained rice in a rice cooker, 1.2 times in a clay pot (donabe) or saucepan, and 1.0 times in a pressure cooker. This guide explains why those ratios work and gives a chart by amount and cooking method, how to adjust for wash-free rice, new-crop rice, brown rice and glutinous rice, how to make rice firmer or softer, soaking times by season, heat settings and timings for a clay pot, saucepan and pressure cooker, the water for sushi rice, seasoned mixed rice and rice with grains, how much rice to cook for a given number of people, and what went wrong when a batch fails, all in concrete millilitres.
Read article →How Strong Is a 10, 15 or 20 m/s Wind? What Each Wind Speed Does to Umbrellas, Walking, Bikes, Cars, Trains and Typhoons, With km/h and Knot Conversions
A forecast of “maximum wind 15 m/s” does not immediately tell you how strong that wind feels. A 10 m/s wind is 36 km/h and already too strong for an umbrella. At 15 m/s you can no longer walk into the wind, at 20 m/s you cannot stay on your feet without holding on to something, and above 30 m/s trees and utility poles start to come down. This guide explains how to read the wind speed in a forecast (mean wind versus gusts, and the height it is measured at), gives a chart of what happens to people, trees, cars and buildings from 1 to 40 m/s, describes 5, 10, 15, 20 and 30 m/s in detail, and gives guidelines for umbrellas, laundry, cycling, camping, drones, work at height, trains and expressways. It also covers the force on a sign or umbrella in kgf, wind chill, how much stronger the wind is on a high apartment floor, typhoon intensity classes and hurricane categories, converting between m/s, km/h, knots and mph with mental-math shortcuts, and what to do before strong winds or a typhoon, all with concrete numbers.
Read article →