In Which I Use Algebra


In what seems like a former life, I spent some time teaching algebra to pre-teens and teenagers. The number one question anyone who spends time in this particular pursuit hears is, of course, “When am I ever gonna have to use this?” (sometimes accompanied by a curse word, depending on the teenager).

Cat dislikes algebra

Most of them sounded a lot like this.

I usually had to go with “well, you need it for college,” or “it’s helpful for logical thinking practice”, or a cooking or construction example that 95% of the students wouldn’t be able to relate to at all.

But I was thinking yesterday… geez, I wish I had been designing then. Because I use algebra ALL THE TIME now. I have a whole notebook full of algebra equations. It’s a thick notebook, even. I use algebra to figure out sweater dimensions, to upsize and downsize a sample so it’ll fit people of many shapes and sizes, and to calculate the exact ratio a sleeve cap should have to its corresponding armhole.

Here’s what I was doing yesterday when I started thinking about this: working out yarn requirements for the different sizes of my¬†Bevin Pullover.

First I split the pieces of the sweater up into geometric shapes:

geometry of a sweater

That says "sleeve", not "Steve". Trust me. I'm a professional.

Then I wrote out algebraic formulas for the area of each shape, in order to figure out the square inches of knitting in each size:


I'm especially proud of myself for remembering the formula for the area of a trapezoid without having to look it up.

It might look a little complicated, but it’s actually very basic. I just took my numbers for each size, plugged them into the formula, and it spit out the total square inches for each size. Then I figured out how much yarn per square inch my swatch used, divided by that number, and voila – fairly accurate yardage amounts!

If only those kids could see me now….


  1. Pumpkin

    That is such an eye-opening idea. I very rarely use algebra either, except for when I had to review it for the GRE. But it would be so useful in this case. I would really love to hear how knitting up a project would come out, I’m really curious. Thanks for sharing!

  2. Michelle

    I was just joking with my husband about how OFTEN I balance equations! Like, anytime I buy yarn for a sweater, I’m doing math! I never believed my teachers when they told me I would need the stuff (math, that is).

  3. Sean

    As one of those students, I have to say, that’s totally beastly hahaha. Oh and I use basic algebraic equations in the kitchen every day :)

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