You're supposed to add 7/8ths of an inch to your finished size to make half square triangles. But they must be sewn very accurately or they'll be a little small. So add a full inch and then square them up. So if the finished size you need is 3 inches cut them 4 inches. You'll then square them up to 3.5 inches and when sewn into your block they'll be 3 inches.