Already have some yarn?
Stashcraft's Yardage Check adds up the yarn of that weight in your stash and tells you whether it's enough for your pattern. It also counts your rows from the Lock Screen and keeps your patterns in one place.
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Questions
Why do thinner yarns need so many more yards?
Thin yarn makes small stitches, so it takes far more length to cover the same area. The same sweater in fingering weight can need twice the yardage it needs in worsted.
Why buy extra?
Yarn is dyed in batches called dye lots, and two lots of the same colour can look noticeably different side by side. If you run short and buy more later, it may not match. A little spare from the same lot avoids that, and leftovers are always useful.
Does crochet use more yarn than knitting?
Usually, though not always. It depends on the stitch and how tightly you work. Adding a quarter to a third is a common rule of thumb. If you have a crochet pattern, its yardage is more reliable.
My pattern says something different. Which is right?
The pattern. It's written for an exact size, gauge and stitch, so trust it over any general estimate. Use this calculator when you're planning, browsing a yarn shop, or deciding whether a sale is worth it.
Should I buy by weight or by length?
By length. A 100 g skein of fingering yarn might be 400 yards, while 100 g of bulky can be closer to 110, so grams alone tell you little.