Square in a Square Calculations

16 Broken Sash

There are a number of ways to approach Square in a Square-type blocks. Let’s take a quick look at them:
1. Covered Corners
2. Calculation
3. Estimation
4. Drafting & Foundation Piecing or Using Templates

It’s then up to you to use the method that most suits you!

1. Covered Corners

Consult the tutorial.

While this method is extravagant when it comes to fabric; however, there is nothing in the way of tricky maths, and it’s quick and accurate.

2. Calculation

The annoying thing about Square in a Square is that while we often know the size we want for our finished block, it’s a bit tricky to work out the size of the central square. Here’s the rule:

The rule for working out the size of the central square in Square in a Square:
Multiply the finished block size by 0.7071,
round to the nearest 1/16″,
then add ½” seam allowance.

Now, we have our central square, how big do we make those triangles? The quickest way is to make the triangles is to take a square and cut it into quarters across the diagonal.

The rule for working out the size of the square to quarter for the edge triangles in Square in a Square:
Multiply the finished size of the central square by 1.4141,
round to the nearest 1/16″,
then add 1½” seam allowance.

I have worked out these rules using Pythagoras’ Theorem. I love maths! Here’s further information, if you’re interested.

If you have a block like Economy, which has a square in a square in a square:

18 Economy

Just use the rules above to work out the size of the squares to cut, but use the measurement before you round and add seam allowance.

If all this is still too hard, then do a quick search for websites that do the calculations for you. Some have tables of values, others have apps. Alternatively, EQ7 and programs like it do all the calculations for you.

3. Estimation

In this method, you’ll still need to know the size of the central square, using the calculations above.

When it comes to the corner triangles, instead of calculating the size of the square to quarter, just use your quilter’s intuition to cut a square a bit bigger than needed. I use this method all the time.

In teh challenge, though, I’ll give you exact sizes, so you won’t need to waste any fabric.

4. Drafting

When you know the size of the finished block, it’s a simple matter to draw up a square, divide the sides in half and fill in the lines! Tricky calculations are completely avoided. You can then take your drafted square and use it as a foundation for foundation piecing, or for the production of templates.

Those techniques are for another blog entirely …

Square in a Square Piecing

If you are cutting a square to make triangles:

Make your first cut:

diagonal cut 1

Then, without moving the fabric, make your second cut:

diagonal cut 2

This way, you have the corners with which to align the ruler.

Once you have calculated and cut your triangles, it is a simple matter to apply them to the central square. Centre the triangle, right sides together:


Sew with a scant ¼” seam. Repeat on the opposite side. Press towards the triangles. You should end up with something like this:

first couple

Now centre the next two triangles over the square, as you did the first:

next couple

Sew with your scant ¼” seam. You should bisect the angle between the new triangle and the one already attached:


Again, press the triangles towards the outside. You will end up with this:

sis fini

Trim the ‘ears’, and it is finished!