Why wool weight and not the length? Why are balls of wool/yarn sold in grams (or back in the day ounces) and not by metres?
Well.. do you want to measure out the whole ball? Wool weight is a much easier thing to measure and is just as accurate. Many of the ball-bands indicate how much length you can get for your weight and some patterns will also indicate how much yardage or weight you need to complete a pattern. It might take some calculations but it’s easier than trying to measure the ball.
One of my most used tools for the creation of the knitting kits is a weighing scale to make sure each kit has enough yarn to complete the project.
Most balls/skeins these days are sold in either 25g, 50g, 100g, 200g and the other day I got a whopper of a ball at 400g.
A lot of the lace weight very fine yarn will come in 25g skeins because a little goes a long way. Chunky yarns come in either 100g or 200g, but don’t forget – because it’s fatter it wont go as far as 100g of double knit or Aran. Knitting a chunky scarf takes a lot more balls (and yarn!) than finer scarves simply because it is so bulky.