Yardage is the number of yards you get in the skein or ball. It is the most useful metric we have when we look at substituting yarns. So if you had a blanket pattern that called for 7 x 50g balls of merino wool but you wanted to use a cotton instead just buying 7 balls wouldn’t be the right thing to do because cotton is much heavier than wool and you may not have enough to finish the project. To work out how many balls you needed you would calculate the total yardage thus:
(Balls needed x yardage of specified yarn)/yardage of chosen yarn = balls needed
be sure to round it up not down.
So taking the example above it might look like this:
(7×125)/85=10.29 so you would need 11 balls.
When designing or working without a pattern being able to calculate your estimated yardage is essential for buying the right amount of yarn. After being caught out a few times despite careful calculations I am wondering if that is art rather than a science though!