Math + Sewing ~ Fractions + Seam Allowances!

on May 10 | in Sewing + Quilting Tips | by | with 11 Comments

Laura Laing, author of Math for Grownups (the book + the blog!), loves to share easy tips and reminders to help adults use math effectively in every day situations. Sewing is chock-full of math, of course; Laura joined us on the blog before to talk about Spatial Skills and Patterns. Today she’s back to share a little about fractions. Learn about Laura in her introduction, and stop by the Math for Grownups blog for more!

If it weren’t for fractions, sewing would be a heck of a lot easier. Am I right?

But here’s the good news: almost all of the fractions used in sewing follow very predictable patterns. That means you don’t have to depend on the math you learned in elementary school— You know, finding a common denominator or converting improper fractions into mixed numbers. If you haven’t used those skills in a while, you’re probably pretty rusty.

And here’s another neat thing to notice. If you read music, the fractions are exactly the same. Cool, eh?

The key to understanding sewing and music fractions is to think of them as parts of one another. Take a look:

½ + ½ = 1
¼ + ¼ = ½
â…› + â…› = ¼

So it’s probably pretty easy to remember that ½ plus ½ is 1. If you think of that same pattern for ½, you come up with ¼ plus ¼, right? You could create a list of these sums that goes on forever.

But the gist of it is this: two ½s is the same thing as one; two ¼s is the same thing as ½ and two â…›s is the same thing as ¼, right?

Now, take a look at how this works out on a ruler or an adjustable seam gauge.

Count the space to the left of the ½ mark and the space to the right of the ½ mark. You’ll get two ½s , which is the same as 1. Easy-peasy.

Now do the same with the ¼s above. There are four of them, right? Two of them make ½, and four of them make 1. Now for the â…›s.

There are eight â…›s in the 1 inch, four â…›s in each ½ inch and two â…›s in each ¼ inch.

Make sense? Now you can use this information to help you deal with seam allowances. For this, let’s look at an example.

Suppose you’re making a pillow, like this Criss-Cross Cushion. To find out how much material you’ll need, you have to consider your seam allowances. In fact, you’ll need to add the total seam allowances to each dimension of the pillow.

But what is that total? If you’re using a ⅝-inch seam allowance, you’ll need to double it for the width of the pillow and double it for the height. That’s because you’ll have two seams that equal the width of the pillow and two seams that equal the height of the pillow.

But doubling ⅝-inch is a real pain. I’m here to tell you that you don’t need to multiply or find a common denominator or change an improper fraction to a mixed number. (Thank goodness!) Instead use the fraction facts above to help.

(Disclaimer: If you want to multiply or find a common denominator or change an improper fraction to a mixed number—go right ahead! The beauty of math is that there are often many different ways to come to a correct answer. Use the process that works for you.)

First off, it’s important to know that ⅝ is â…› more than ½. How do you know that? Take a look at the picture below. If you count to ⅝, you’ll see that you’re â…› above the ½ mark, right?

In other words:

½ + â…› = ⅝

Now remember what you already know.

½ + ½ = 1
â…› + â…› = ¼

That means that when you add ⅝ and ⅝, you’ll get 1¼. Here’s how that’s done, mathematically:

⅝ + ⅝ = ?

Substitute what you know about ⅝:

(½ + â…›) + (½ + â…›) = ?

Now rearrange the addition, so that’s it’s easier. This is possible because you can add numbers in any order you’d like:

(½ + ½) + (â…› + â…›) = ?

Finally, it’s time to add:

1 + ¼ = 1¼

So every time you need to double your seam allowance, you’re adding 1¼.

Experienced sewers already know this. But the process that we just went through can be really useful when you’re adding any sewing fractions. Try it with ¾ + ¾:

¾ + ¾ = ?
(½ + ¼) + (½ + ¼) = ?
(½ + ½) + (¼ + ¼) = ?
1 + ½ = 1½

Ta-da! Now you can add any sewing fractions with ease, making fancy French seams or a variety of unique quilting patterns.

Pin It

Related Posts

11 Responses to Math + Sewing ~ Fractions + Seam Allowances!

  1. Wendy R says:

    To those who think that was too easy for adults, think again. Thank you for the post, some of it actually made sense to me.

    I am 54 and math and I never got a long together. It is a foreign language I do not understand. I am art inclined and think in a more abstract way than math would allow me to do. I am sure I am not the only intelligent adult who just didn’t get it. I now get a little of it, so again, thank you! 🙂 (By the way, my dad was a Physics and Math teacher, haha).

  2. Peter Price says:

    @Suzanne & Sarah, as one who teaches mostly young adults who are studying to be primary/elementary teachers, I can tell you that many of them are confused about fractions.

    I wish that the math on this page was too easy for all adults, that they learned this before they turned 10 or 11 years old, but it simply isn’t true. If you find this math really easy (I’m with you on that one), be thankful! Perhaps we can join Laura and do our bit to help others understand the beauty and simplicity (as we see it) of math!

  3. kathyh says:

    Sewing math – love it.

  4. Connie says:

    I’m a little shocked that anyone would have to go through all those steps to get that 3/4 plus 3/4 equals 1 1/2… really? Adults shouldn’t even have to think that through, they should just know it. Same with knowing that 5/8 is more than 1/2… of course it is, because 4/8 is a half, and 5 is more than 4… geeze!!! If adults really need this kind of help with simple fractions, I’m scared for our future!!

  5. Traci Sue says:

    I work in a fabric shop so I do math like this all day. It seems to truly amaze people when I can do this sort of thing in my head.

  6. Sarah M says:

    Agree with Suzanne above.

  7. Suzanne says:

    I think this is a little simplistic for adults.

  8. Thanks for sharing this website. I am a math teacher who loves to sew! Great to have some new things to use to turn kids on to math.

  9. Carol says:

    This is such a timely post…just today as I was working on a large quilt top I was thinking how sewing/quilting is the only math I can do without thinking about it being math. 🙂
    That was…until I saw the math problems above, then I got a case of hives! :/
    It’s just something about numbers….

  10. Nothy Lane says:

    Hi there,

    I love this book! I am going to order a copy asap. I teach math and I sew a lot. I have been looking for a reason to have a sewing club at school!

    Nothy Lane

  11. dana says:

    Excellent! Can she do a remedial geometry tutorial? LOL…so many questions about cones…. I need her book!


« »

Subscribe to the newsletter

Sewing inspiration, projects, events and offers delivered conveniently to your email.


Get the latest news via