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Seam Allowances for Half Square Triangles & Quarter Square Triangles Are Not What You Think

A 1/4" seam allowance is most commonly used in quilting.  

But when you use a quilt ruler to cut squares and triangles, you can't just add 1/4" to the strip.

To make Squares, you need to add 1/2":

To make Half or Quarter Square Triangles,
you need to cut Squares first.
Then, cut them diagonally in half or in quarters.

If you only add 1/2", you won't have any seam allowances on the diagonal edges.

So, these are the seam allowance amounts patterns use today:

But those amounts are not exactly right!

Ever Wondered Why

you have to trim your triangle squares down to the right size?

Triangles come out slightly larger than squares.

You Might be Surprised

to hear that you can't add the precise amounts of seam allowances for triangles, using a quilt ruler alone.

That's Because

Rulers only have lines every ⅛”.

  • 7/8" is almost 1/32" too much for HSTs
  • 1¼" is almost 1/16" too much for QSTs

This is why many quilters today add even more
and then trim them down to the right size later.

Believe it or not, the exact amounts are:

You could use individual templates or
dies to cut perfect triangles.

But you'll need a separate template or die for each size you want to cut.

And that can get really expensive!

Cut accurate triangles  

Any Size
No Expensive Dies!


Seam Allowance Additions

let you add the precise amounts of seam allowance
for triangles right on the edge of the ruler.

The Green H Addition
adds 0.854" for HSTs.

The Red Q Addition
adds 1.207" for QSTs.

When you add the accurate seam allowance amounts, your squares and triangles will match perfectly.

See how it works in the animation below:



Seam Allowance Additions

come in two versions.

Guidelines Ruler Version

The Guidelines Ruler version locks into holes in the corners of the Guidelines Ruler.

Regular Acrylic Ruler Version

The regular acrylic ruler version attaches with 3 suction cups to the edge of any standard 1/8" thick acrylic ruler.

Our complete Finished-Size Quilting sets include
Seam Allowance Additions and the Prep-Tool that lets you sew
accurate scant 1/4" seam allowances.  

Check out the options below.

Click here for set options with 1, 2 or 3 Guidelines Rulers.

Click here for set options that you can use with regular 12" & 24" rulers you already have.