There’s a great many items we make in crochet that have circular construction. We want them to fit. It’s easy to measure the head or body circumference, but it’s hard to measure the circumference of a flat crocheted item. We can measure the diameter of the item, but it’s hard to measure the diameter of your head. Unless you have one of those old-timey calipers that prenology people used.
Yep, that’s weird. So, instead we have to be able to go from one measurement to the other. Our friend is Pi. Not pie. I mean, I love pie, too, but we have a troubling relationship. It’s not exactly what I’d call a friendship. But I digress.
Pi. The value of the ratio of a circle’s circumference to its diameter. It is a neverending decimal. I have to tell you, I went to the Texas Academy of Mathematics and Science. It’s a high school program at the University of North Texas. Basically we were living in a dorm on campus and going to college classes during what was our Junior and Senior years of high school. We’re a bunch of nerdy overachievers. Some even made up a cheer that was something like “3.14159, tangent, secant, cosine, sine!”
Again, I digress.
If you have the circumference of the item (someone’s head for a hat), then you need to divide by Pi (3.14 is enough), then the resulting number is the diameter. Crochet an item until it is that diameter (or slightly less) to make sure your sides will fit around the item.
When you’re crocheting a circle, if you’re trying to decide when it’s big enough for your head, measure the diameter (distance from one side to the other across the center), then multiply by Pi. The result is what the circumference will be if you stop increasing the circle at that point.
Any questions? You’ll need to take into account all yarn fiber and stitch information when determining the actual measurements of your item.