What's the math?
 

I need a couple of 1/4 square triangles for fillers. I changed a pattern to standard sizes as I wanted to try out a couple of Sizzix dies. The squares in the pattern were something like 2 5/8 to cut or another weird size.

I changed to 2 1/2 - Slow going to fix the changes but working. I know you need to add so much to a square to cut triangles. What do you need to add if you want quarter triangles? I cut the triangles on a Sizzix so I'd have to have the formula for quarter triangles for a 2 1/2 cut or 2 inch finished block. Cutting the half triangle is a little short.

Next time I'll shove the design in Electric Quilt and let IT do the math!

I'm just a trifle annoyed with odd sizes designed to get you to buy new rulers.

You need to add 1 1/4" to your finished size but I usually add a bit more and trim my triangles to get a more exact triangle. For example for HST you are supposed to add 7/8" and I always add at least 1" and trim. For QST I'd add 1 1/2". There is some waste but very little and it is worth it in the end.

I agree. I need that little bit "extra" in the cut piece. I ALWAYS end up with skimpy units when I cut the square to the "exact" mathematical requirement. I would much rather trim off 1/32 to 1/16 of an inch and have an adequate unit than have it be short that much.

Those "odd sizes" are not designed by anyone. It's just the mathematics of the situation. And adding 7/8" or 1.25" can be done with any ruler. But I agree with the others - I add a bit more and trim down, because my seam is seldom exactly at the right place.

Where do you add the 1 1/2? Sounds weird, but I cut a bunch of extra 2 1/2 inch squares and would like to cut the 1/4 triangles from those. I also have a lot of HSTs. How would I cut those down? If I make a template out of the size I need, can I just add 1/4"? I don't need that many - 10 or 12 at the most. The rest are HSTs and already cut