Quilt Blocks: Simple Math Part 2

Last week we began looking at some quilt blocks and learning how to figure patch sizes. You might want to review that post, Simple Math Part 1.


150Blocks Quilt Blocks: Simple Math Part 2


When you’re figuring patch sizes, the first thing is to determine how a block is divided. You need to know how many equal divisions there are across the block. Here’s an example.

blockbonanza2 Quilt Blocks: Simple Math Part 2

There are five equal sections or “columns” across this block, which you can see by the black lines I’ve drawn. This is the first piece of information you need: 5 divisions.

If you know the finished size of the block, you will begin to figure the math by dividing the finished size by 5. If this is a 10″ block, that means:

10″ ÷ 5 = 2″

This tells you that each of the divisions is 2″ finished.

lavenderblock Quilt Blocks: Simple Math Part 2

Now you can figure out how large to cut the patches. Let’s start with the easiest ones first which are the squares.

mgblock2 Quilt Blocks: Simple Math Part 2The larger purple squares take up one column, which we know is 2″ across. A square has equal width and length, so you know the finished size is 2″ x 2″. You’ll need to add 1/2″ for seam allowances (that’s 1/4″ for each side, times 2 sides, equals 1/2″) so you’ll cut the patches 2 1/2″ x 2 1/2″. You can see that you’ll need 5 of these squares.

It’s worth mentioning here that you don’t worry about seam allowances until you have a finished patch size. Only then do you add the 1/2″ for seam allowances.

mgblock4 Quilt Blocks: Simple Math Part 2

What about the smaller purple squares? You can see above that two of them take up the 2″ width of one column. Again, this is simple math. If you know the column is 2″ wide and there are 2 squares in it, how wide is each one?

2″÷ 2 = 1″

Each small purple square is 1″ x 1″, plus 1/2″ for seam allowances, so the patches are cut
1 1/2″ x 1 1/2″. You’ll need 16 of these squares.


mgblock5 Quilt Blocks: Simple Math Part 2Look at the block again and you’ll see there are also small yellow squares which are the same size as the small purple squares. You’ll need 16 small yellow squares cut
1 1/2″ x 1 1/2″.


mgblock6 Quilt Blocks: Simple Math Part 2Next we’ll figure out the rectangles. Look at the diagram above: these are still easy to determine. Two rectangles (outlined in black) cover one square that we know is 2″ wide, and also 2″ long. Logic tells us that the long side of this rectangle must be 2″ finished.

We figure the short side with simple math.

2″÷ 2 = 1″

The short side of the rectangle is 1″, so the finished size is 1″ x 2″. Add 1/2″ for seam allowances and you know to cut the rectangles 1 1/2″ x 2 1/2″. You’ll need 4 green rectangles and 4 yellow rectangles.

MH900330968 Quilt Blocks: Simple Math Part 2

Now—don’t panic—we’re going to figure out the triangles. There are two simple things to know about figuring triangles for quilting. Today we’ll talk about just one of them.

It’s called the 7/8′s rule.

Whenever you need a triangle that is half of a square, like this:

mgblock8 Quilt Blocks: Simple Math Part 2

You take the finished size of the square and add 7/8″ to get the cut size. That’s it. No seam allowances, nothing else. Just the finished size of the square plus seven-eighths. Cut a square that size, cut in half diagonally, sew two triangles together and just like magic, you’ll have a square the correct size.

mgblock8 Quilt Blocks: Simple Math Part 2

We already know that the width of this section is 2″. So it’s simple math from there:

2″ + 7/8″ = 2  7/8″

You’d cut squares 2 7/8″ x 2 7/8″, cut them in half diagonally, and sew the resulting triangles to matching triangles of a different fabric to get a half-square triangle, also called a triangle-square.

For this block, you’ll need to cut 4 of yellow and 4 of pink/orange. Why only four when you see eight triangles? Because you’ll cut them diagonally in half, which gives you 8. Make sense?

Lots more simple math to come in Part 3 next week. Stay tuned!

Do you have any questions so far? Ask away in the comments and I’ll do my best to answer.

About Diane Harris

I'm an editor for Quiltmaker magazine in Golden, Colorado, USA. For six years, I wrote pattern instructions, product reviews and how-to articles. Then I spent four years as QM's Interactive Editor, working to generate much of our online content. Now I'm back to patterns and how-tos, which is a great fit for me. I still love writing about quilt-related topics for Quilty Pleasures, and I always have my finger on the pulse of the quilting world. I teach a variety of quilt classes and give guild programs, too. Reach me by email: editor@quiltmaker.com.
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8 Responses to Quilt Blocks: Simple Math Part 2

  1. camille woods says:

    I’d like to ask, why do we add 7/8 and not 1/2, when making triangles? I wonder who figured it out and is the 7/8 used any where else in quilting? thanks, camille woods

  2. maria Cristina says:

    uma explicação muito bem detalhada, que não nos deixa dúvida alguma para saber o tamanho de cada quadrado ou tiângulo com as costuras incluidas ,muito bom mesmo

  3. Pingback: Quilt Blocks: Simple Math Part 3 | Quilty Pleasures Blog

  4. Sewing Sue says:

    This is such good information! We need to find a new work for “math” that doesn’t scare people… hmmm… wonder what we could call it?

  5. Bev Gunn says:

    Jodi, link is at the end of the first sentence! Very interesting and informative!

  6. Jodi Haskell says:

    Could you give me a link to part one? or could you email me a copy? this is one thing that is hard for new quilters! thanks. Jodi

  7. Diana says:

    thanks! This has been great so far. And it didn’t hurt too much.

  8. Christine Slaughter says:

    I have such a hard time with math. In fact, it’s so bad that the kids know to go to their step-dad with math homework…LOL This is SO helpful!! Thank you so much!! :)

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