Thursday, October 30, 2014

Quarter Square Triangle Formula

How large should you cut your squares to make quarter square triangle units of any size?

If you are a very precise cutter and piecer, then cut your squares 1 1/4" larger than the FINISHED size of the quarter square triangle unit you need.

The exact formula is FINISHED size plus 1 1/4". You need to be very accurate when you cut and sew your pieces for this to work. So I cut my squares a little larger and then trim the units down to the correct size.

Lynn's formula is FINISHED size plus 1 1/2". If you are short on fabric, adding just 1 3/8" will be plenty, but I don't like having to fool with eighths if I don't have to.

Example: Suppose you need your quarter square triangle units to finish at 2" square. Then cut two squares (light and dark) at 3 1/2".


Draw a diagonal line on the back of the lighter square. You could also fold one of the squares in half diagonally and press the line instead of drawing.


Put your two squares right sides together with the lighter one on top and sew 1/4" away on both sides of the drawn line.


Cut on the drawn line.


Press the seams to one side or open, which ever you prefer. However, if you press them both to the darker fabric your seams will nest nicely when you sew again. You'll end up with two half square triangle units at this stage. However, we want quarter square triangle units so we need to continue.


Flip one of these over and draw another line on the diagonal that is perpendicular to the sewn line of the half square triangle unit. Another way to think about this is that your drawn diagonal line needs to cross the previous seam you sewed and be half on the light fabric and half on the dark, see photo below.


Again put your two squares right sides together. Line up the pressed seam and make sure that the dark fabric is underneath the light fabric and the light fabric is underneath the dark. I offset the squares slightly in the photo below so you could see how they need to line up.


Notice how the seams go in the opposite direction because I press them both to the darker fabric. 


Sew 1/4" away on either side of the drawn line.


Cut on the drawn line.


Press and now it is time to trim these down to the correct size. Lay one of the quarter square triangle units face up on your cutting mat. You'll need a square ruler that is larger than your unit, mine is a 4" square. Place your ruler on top of your unit so that the 2 1/2" lines are on the square and the diagonal line on the ruler is on the sewn diagonal line of your unit.


In the photo above, notice that the 1 1/4" lines meet right in the center point of my unit. We will be squaring this up to 2 1/2" and 1 1/4" is half of 2 1/2".

Trim off the excess on the right hand side and top side. Then rotate your unit 180 degrees. This time when you put your ruler on top, the 2 1/2" lines should be on the two sides you just cut. Again, make sure the diagonal on the ruler is on the diagonal of your unit and the 1 1/4" lines meet in the center of the unit.


Trim again along the right hand side and top. Repeat with the other quarter square triangle unit. Now you have two 2 1/2" quarter square triangle units. When you piece them into your project they will finish at 2" square.


If you have any questions, please ask them in the comments.


P.S. This is a repost from my LQS blog. Since my LQS is closing, that blog will be disappearing. I was the main contributor of that blog and I wanted to keep the tutorial posts I did for it. So I am reposting them here.

1 comment:

West Michigan Quilter said...

This is so good to know. Thanks for the tute. I'm sure I'll use the 1 1/2" addition.