Photo CC-licensed from Flickr User Net_Efekt

Yesterday, designer Ellen Gormley asked a great question (via Facebook and Twitter): “crochet/knit designers: What’s your best method for measuring the circumference of a hat?” And when I gave my answer, I thought–“oh, I should share that here on my blog too,” because it’s a calculation I use a lot and you might find it useful too.  Yes, this is math and may require a calculator, but don’t worry because it also involves Pi, and who doesn’t love Pi?

CC-Licensed photo by flickr user pauladamsmith

So, say you have a hat you love, and you want to knit or crochet one that is the same size and shape. How do you figure out the size of the hat? You could measure the circumference (the distance around the hat) with a flexible measuring tape, but this is tricky and probably inaccurate as the fabric and the tape will slide around a lot while you’re doing this. (In fact, if you insist on this method, put the hat on some sort of hard form, like a head, while measuring).

Instead of trying to measure the circumference, I lay the hat flat, so that the whole crown of the hat is visible on a hard surface, and I measure the diameter–the distance across the crown of the head. With that measurement, I can then do a little calculation and find the circumference:

c = πd

C is the circumference, π = (for our purposes) 3.1416, and d = the diameter. So say you measured 7 inches across. Your calculation would be

c = 3.1416 (7) = 21.99 (or 22″)

Then you can take this measurement and, using your gauge swatch (because of course, you swatched right?) You can figure out your foundation chain or your cast on for your hat. (If you’re working from the crown down, you can just crochet until your hat’s diameter matches the size you’re trying to achieve).

Easy as Pi