One of the questions that comes up a lot on Ravelry is "I have a cone of yarn that weighs this much... is it enough to make a sweater?"
The general rule of thumb is that a sweater takes about 1500 yards, but the fact of the matter is there's no way to give any reasonable answer without having a lot more information. Not only does the yards per gram differs among fibers, the number of yards in a 1-pound cone depends on the size of the strand and in some cases, the spinning system used to manufacturer the yarn.
However, there is something you can do to get a pretty good estimate.
First, you must knit a swatch. That shouldn't be a problem because you always knit a swatch anyway, right?
So knit a swatch thats big enough to get accurate measurements -- about 80 stitches by 100 rows for finer yarns and 50 stitches by 60 rows for bulky yarns.
After washing and blocking the swatch the same way you'll finish the completed garment, measure the length and width. You'll also need to weigh the swatch to determine how much yarn it consumed.
Now you have all the information you need to determine the amount of fabric you can knit with the amount of yarn you have.
It'll be easier to explain with an example, so grab your favorite calculator and follow along.
I have a cone of lovely but unidentified yarn that weights 600g. Most paper and plastic cones weight about 25 grams, so I really have about 575 grams of yarn.
My swatch is 8 inches wide, 4.5 inches long, and weighs 13g.
My sweater is roughly 38 inches around the body and 22 inches long. We don't have to be exact here... rough measurements are ok. When in doubt, add an inch or so in both dimensions.
My sleeve is about 13 inches wide at the widest point and 23 inches long. Again, rough measurements are ok.
Let's calculate the yardage for the body first. To make things easier, we're going to work in one dimension at a time -- first width, then length. I also want to err on the side of caution, so I'm going to round up to the nearest third or quarter (.25, .33, .50, .66, .75).
- The body is 4.75 times the width of my swatch (38 / 8 = 4.75), so a 38 x 4.5 rectangle needs 62g (4.75 x 13g).
- The body of my sweater is almost 5 times the length of my swatch (22 / 4.5 = 4.88), so a 38 x 22 rectangle needs 310g (5 x 62g).
Now let's calculate the yardage for the sleeves using the same technique.
- The sleeve is about 1 2/3 times the width of my swatch (13 / 8 = 1.625), so a 13 x 4.5 rectangle needs 22g (1.66 x 13g).
- My sleeve is more than 5 times the length of my swatch (23 / 4.5 = 5.11), so a 13 x 23 rectangle needs 116g (5.25 * 22g).
- A sweater isn't of much use with only one sleeve, so we need to double that to 232g (2 x 116g).
Now, we just add up the numbers to get the total yardage requirement: 310g + 232g = 542g.
Here's a little cheat sheet you can print and keep with your yarn so you'll have it handy the next time you need it:
Knitting Yardage Worksheet
|A = Swatch Width:|
|B = Swatch Length:|
|C = Swatch Weight:|
|D = Desired Width:|
|E = Desired Length:|
|F = (D / A) * C:|
|G = (E / B) * F = Total Yardage Required:|