Sunday, March 14, 2010

Pi in the Sky

Thank you all for your nice comments on Wisteria and for the tips. I'm going to keep working on it and will let you know how it goes.

This is just a quick post to wish you all a happy Pi Day. For those less geeky than I and not already familiar with the "holiday", Pi Day occurs on 3/14. Pi is the ratio of a circle's circumference to its diameter, and today serves as a convenient reminder to think about how important this number is.

For knitters, pi allows us to estimate how much yarn we need in doing a long-tail cast-on. If you visualize a knit stitch as basically a circular loop (more or less), then the amount of yarn needed to form the loop (its circumference) is about 3 times the width of the loop (its diameter). So if you are casting on for a piece of knitting that is going to be 10" across its width, you can estimate that you'll need a bit more than 30" of yarn to cast on. Of course, you'll need a some more for the tail itself, especially if you plan to use it to seam, but the "3x" rule is a good starting point.

Since pi is a constant, when the diameter doubles, so does the circumference. If you've ever knit some variation of EZ's pi shawl, then you've put this to good use. This is the reason that if you doubled your stitch count on row 3, then you'd do it on row 6, then 12, then 24, and so on! Sometimes when you knit a circle, the pattern will have you increase your stitch count more subtly, by tucking in a smaller number of increases on every round or every other, instead of doing them spaced out so far apart, but if you wrote out the stitch counts for each row, you'd see that this same relationship is maintained: double the row number, double the stitch count.

I am anxious to start working on a circular project of my own, but it's still percolating in my head. I'll be sure to show you if something comes of it.


  1. that was so neat, I had not done much reading about the pi shawl or about the ratio in relation to long tail cast on (my cast on method of choice, so you'd think I'd know!) This was great, thank you.

  2. Clever! I'd never thought that much about the "rule of 3" for long-tail cast-on... or 4, I guess, if you're casting on more stitches, as that extra 0.14 adds up.