One of my favorite quilting tools is the Bloc-Loc ruler for half square triangle blocks. Most patterns will tell you when making a 3″ finished half square triangle block to cut your squares 3 7/8″ or to make a 2½” finished half square triangle block to cut your squares 3 3/8″. I always round up those measurements 1/8″ so I cut 4″ squares for a 3″ finished half square triangle block or 3½” to make a 2½” half square triangle block. When you cut your squares this 1/8″ larger, you give yourself the opportunity to square up your blocks so they fit more easily in with the rest of your quilt’s pieces.
There is more than one ruler designed for squaring up your blocks but my favorite ruler for half square triangle blocks is the Bloc-Loc ruler. This ruler has a ridge cut out that is exactly on the diagonal and offset to one side so it fits over the seam of your half square triangle block. You place this ruler so the ridge cutout is over the seam with the diagonal of the ruler on the sewn side of the seam. When placed on your block in this manner, the ruler will literally lock onto your seam and slide up and down on it. So if you are trimming up your 3″ finished half square triangle block, you will want to slide the ruler on the seam so you have a little bit to trim off all around the desired 3½” unfinished half square triangle block size. Trim off the 2 sides of the block that are away from you, rotate the block and ruler 180°, line up the sides you just cut with the 3½” markings on the ruler, and trim the other 2 sides.
Once you get the technique down, you will find using this ruler will make your half square triangle blocks very accurate and it won’t take you near as long to square up your blocks. Believe me, I’m all about faster and more accurate at the same time.