# Why Does the 1/2 Square Triangle Formula Require You to Add 7/8 of an Inch?

Learn why quilters must add 7/8 of an inch to the triangle’s finished size in order to have enough fabric for ¼ inch seams.

CUTTING AND SEWING HALF SQUARE TRIANGLES – Why Does the ½ Square Triangle Formula Require You to Add 7/8 Inch? (#3 of 8 videos)

Transcript:
Quilters often find it puzzling that you have to add 7/8” seam allowance. So let me explain that. I have a little graphic here.

If you take your square and you add a ¼” seam all the way around and then you cut it in half on the diagonal, you’ll see that this center part doesn’t have any seam allowance. So you’re going to have to figure that you will add ¼” and this ¼”, ¼” and this ¼”. So it looks like it would be four quarters of an inch which would be an extra inch. However, because these are on the diagonal they’re just a sliver short (when you measure) from being ¼”. And it works out that these two measurements together add up to 3/8 of an inch.

So we’ve got a 1/4″, plus 3/8” and plus ¼” and when you add that all together you get 7/8”.

You can find more information about cutting and sewing half square triangles at LearnHowToQuilt.com under “Beginner Basics” – “Cutting” and “Sewing”.