When I’m tech-editing patterns, one of the things I need to do is figure out the exact amount of yarn needed to make a garment. It’s one of my favorite parts of the process because it seems a little bit like magic and involves some fun, easy math and my kitchen scale.

The first garment is easy. I weigh the knitted sample and divide the total number of grams by the number of skeins used. If you want to know meters v. weight, you can multiply that resulting number by the number of meters per skein.

The fun part comes when you’re figuring out yarn amounts for the other sizes. We all learned how to do this in junior high school algebra class, but until I needed the skill for crocheting I had lost it in the reaches of my brain.

You set up cross products by turning what you know into a fraction, i.e. take the bust measurement of the garment you have and make that the numerator (the top number). Take the total number of grams and make it the denominator (bottom number) like this

32

486

The other fraction uses the garment you’re trying to figure out the yarn amount for as the numerator and “x” as the denominator. We’ll find for “x”

38
—-
x

So the calculation looks like this:



32 38
--- = ---
486 x

Do you remember what to do next? Maybe this looks a little familiar and you just have to shake the cobwebs away for a moment.

1) Multiply 486 x 38. (18468)
2) Divide 18,468/32 (577.1)

Now you know you need 577.1 g of yarn for the 38″ garment.

3) Divide 577.1/50 (or however many grams are in each ball of yarn you’re using).

You’ll need 11.5 (or 12) balls of yarn (of course it wouldn’t hurt to buy 13 just in case your tension varies).

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Down to the yard…

13 thoughts on “Down to the yard…

  • March 18, 2007 at 12:41 pm
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    Amy,

    Thank you so much for sharing this. I was figuring this out a much more complicated way. Once in a while you’ve just got to love the math.

    Thanks,
    Lauren

  • March 18, 2007 at 1:09 pm
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    Ooh, I shall bookmark this post cause you know I need all the help I can get. Thanks!

  • March 18, 2007 at 2:37 pm
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    I love it when someone finds that the math they learned in late elemetary or jr. high, really does have a use. I tell my students that they will find uses for math in places they never suspects. However, please use the term “cross-products” instead of “cross-multiplication”. There is much confusion caused by the term “cross-multiplication”, you should see students multiply fractions if the term “cross-multiplication” is used. It keeps a lot of confusion from happening if the term “cross-products” is used.

    Thanks for the good work.

    Mark
    Math Teacher

  • March 18, 2007 at 3:06 pm
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    Thanks for the help, and encouragement, Mark! šŸ™‚

  • March 18, 2007 at 4:08 pm
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    Oh yay! What incredibly helpful info. You just changed my life. LOL

  • March 19, 2007 at 5:32 am
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    For those who think in SAT-Verbal rather than SAT-Math ways, try thinking of it as an analogy problem:

    32 is to 48 as 38 is to what? It is solved the same way, of course, but verbalizing the relationship helps me figure out what kind of math to use.

    Very cool post!

  • February 12, 2008 at 8:14 pm
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    This is such a great idea! I wish I would have known about it before. It would have saved me a lot of headaches and frustration.

  • March 7, 2008 at 7:45 am
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    Fantastic info! Thanks for sharing. I never really thought of it that way. By the time I would’ve been done, it would’ve looked like a page out of Einstein’s scribble journal!

  • April 4, 2010 at 5:22 pm
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    So simple, so beautiful. I can't believe I've only just now found your post! Pattern Writing, here I come!
    Thanks!

  • April 5, 2010 at 2:31 am
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    Glad it was helpful, Hannah! Can't wait to see your patterns. šŸ™‚

  • April 15, 2010 at 6:00 pm
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    Thanks, Hannah–I look forward to seeing your patterns!

  • March 13, 2013 at 12:14 pm
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    You can also get the same answer with (new bust size)/(sample bust size) x (sample yarn weight).Ā 

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