Do hst's have to be a specific size?
 

I was reading a blurb about making different layouts with the half square triangle and thought I would put to use the many, many 5 inch charms I have, but the post said to cut them down to 4 inch squares once they were sewn together. Why can't I just leave them the size they are when I have them together since I am using the same size square to start with?

If you are making something for yourself, use any size your little heart desires. Why waste good fabric cutting things down if they fit together as they are? I read patterns the way I read recipes - to get ideas - then do my own thing. It's like that old story of the family pot roast recipe that started out with "cut off the end of the roast." Turns out somebody's gramma had a small pan and she always had to cut some off to make it fit.

As long as the HST will fit with any other components of the quilt, size doesn't matter. If the quilt will be all HST, do whatever you want. If it says to start with 5 inch squares and trim them down once they are sewn, I would say they are instructing you to use the method of drawing a diagonal line on the back and sewing on each side. That yields a HST 1 inch smaller than the original square.

If you make two hst units from two five inch squares of fabric, theoretically each hst unit will finish at 4-1/8 inches square.

Try it and see what happens.

The calculation for making HST is that you add 7/8" to the FINISHED size. So to make 4" finished HST, you would add the 7/8, which gives you 4 7/8. Since most folks don't really want to cut odd size squares, they round up the 7/8 to one inch, sew and then trim and square them down a bit to 4.5" (which finish at 4"). Since you trim all sides of the finished patch, you are only trimming off 1/16". Just a sliver.
So basically, if you want to make 4" finished HST, you cut 5" squares.

lol, I never heard that story, but I like the correlation.

I have always found that you can make HSTs any size you want as long as all your blocks end up the same size. Even if you have some smaller or larger you can add a little sashing to smaller ones to bring them up to a matching size, if that makes any sense.

I have never been one to follow rules.
peace

I heard this story years ago and and I have used it as a teaching tool ever since. I am more creative and analytical because of it. Never be stopped by an opportunity to question why or do things differently from someone else, your own way! It might just be a great innovation and be creative and improve the status quo of any thing you do.

Thanks so much for the reminder of this life lesson.
peace

That is how I would do it. We tend to sometimes get away from the math that is required with things such as cutting from strips and using Thangles where the math is embed into the product. Good Luck.