Cool thing I saw on the internet alert!
Have you ever been working on an equilateral triangle quilt and started dealing with seam allowances (which get weird on triangles) and ended up doing trigonometry to figure out what the finished size of your quilt would be?
Yvonne @ Quilting Jetgirl to the rescue! She made this super easy interactive spreadsheet. Put in the height of your triangles, how many triangles in a row and how many rows, and voila, finished quilt size!