How to Do the Math and Cut 8 Half Square Triangles

How to Do the Math to Cut out 8 Half Square Triangles

We have been asked a number of times, how to calculate the size of the starting pieces of fabric when you want to cut 8 Half Square Triangles at the one time. So here it is. We also have a video below on how to cut the 8 half square triangles.

Calculation for 8 Half Square Triangles

Calculation to work out the size of the starting fabric:

The size of your finished block + 7/8″) x 2 = size of starting fabric

For example if you want your finished block to be 2 1/2″ your calculation will be:

(2 ½” + 7/8″) x 2 = 6 ¾” (we round up to 7″ – see note below)

So the size of the square that you need to start with is 6 ¾”

This will result in 8 unfinished squares that are 3″ each. When you sew them into the project you will be using a ¼” seam which will bring the size of the block down to a finished size of 2½”.

Note:

This calculation is for very accurate sewing. If you are like us and would rather trim the blocks (see our video here on how to do that) then you can round up the size of the square. So for our example, we ended up with 6 ¾”, so we would round that up to 7″. This gives you a bit of wiggle room to trim.

In the following video, we show you how to cut half-square triangles. One of these methods is the ‘How to Cut 8 Half Square Triangles’ technique. It’s a great way of cutting lots of half-square triangles quickly.

Scroll through to the 5-minute mark for the information on cutting 8 half square triangles.