Saturday, June 27, 2009

Half Square Triangle Formula

One question that seems to constantly come up with quilters is how do I know what size square of fabric to start with to end with a certain size square when I sew the triangles back together. Basically they want to take a beginning square - divide it in half diagonally - then sew it back together to come up with another sewn square, smaller in size than the beginning square. The problem comes in when you want to end with a certain size sewn square, let's say 2.5 inches (unfinished and so including a 1/4" allowance for a 2" finished square). How do you figure out how large the first square of fabric should be?

I presented my husband with this problem and being the persistent person he is, he came up with this approximation of the formula:

The formula to approximate the half square triangle is:
A = a + 3/8 inch, where A is the beginning square of fabric and a is the ending sewn square

Or Beginning square of fabric = Desired Ending Sewn Square + 3/8 inch

If you are curious, the more accurate version of the formula is:

A = a + (√2 x .25)

So, for our problem of how large a square to start with if I want an ending square of 2.5 inches,
A = a + 3/8
A = 2 1/2 + 3/8
A = 2 4/8 +3/8 (converting to eighths)
A =2 7/8 inches

1 comment:

Robin in Short Pump said...

I would say that what Pat was asking about is exactly this formula. The way I read her question is that she needed a 2.5" HST (I presumed unfinished, although I don't think she specified that), so she should have added 7/8" to the FINISHED size of 2".

I always have to square mine up which is a pain in the neck because what quilt only has one HST? LOL