It’s the nature of yarn to be sneaky. Some yarn limits itself to basic tricks…tying itself in knots when your back is turned, developing mysterious breaks while sitting safely in your knitting basket, subtly changing color in a way that only becomes apparent once the project is finished. But some yarn has more gumption…some yarn likes to shrink when you’re not looking so you run out of yarn with five rounds to go on your project. And the harder it is to get more of that yarn (say…a small batch yarn, only ever available from one far-away supplier, that’s been discontinued for years), the more likely the yarn is to try these tricks.
To thwart this tendency, you have to deploy yarn’s natural enemy…math.
In this case, that meant knitting several repeats of the pattern (here I’ve got four finished) and then doing some measuring. First I measured what this many repeats weighed (being sure I was accounting for the weight of the needles). Then I measured how tall this many repeats was.
A little bit of math let me figure out both how many repeats I’d want (because I know how tall I like my hats) and what that many repeats would weigh. Then I weighed my remaining yarn. And much to my surprise, I’ll have plenty. That means I can relax and enjoy the rest of the knitting without worrying that the yarn is going to do anything truly scandalous (and will instead confine itself to the usual yarn shenanigans).