the basic square is figured out by adding 7/8th of an inch to the size of the half square triangle you want to end up with..there is also software programs where you can print out sheets of paper with grids on it and then layer with paper and sew then cut on the marked lines or you can buy Thangles from a quilting store which is good; these last two are great if you have alot of 1/2 sq triangles to do instead of just a couple