Unlike with buttonholes in woven fabrics, choosing a button for a knit project isn’t as straight forward as measuring the hole length and picking a button with the right diameter. There are other factors to consider like the stretch of the buttonhole technique used and the elasticity of the yarn itself. Stretchier buttonholes mean that you might need to use bigger buttons than you’d expect, unless you plan on adding a woven backing to reinforce the holes. You can use the recommended button size in the pattern, but it might not be quite the right size for the yarn you chose or the variations caused by different techniques.
All of that sounds frustrating, but choosing a button for knit buttonholes is pretty straight forward. You just need to use the finished buttonhole to find the right size. I often wait until my sweater is completed before I start looking for buttons. I use a button from my stash to figure out what diameter I need, note it down, and then I start shopping. That system works well for a procrastinator like me, but I’m guessing it sounds frustrating for crafters who like to get everything sorted out in advance. There’s an easy solution for that, though. All you need to do is include a buttonhole on your gauge swatch. Not only does that give you a way to test out button sizes, but it’s also nice and portable, so you can carry it with you while you shop.