1. The square is 12 1/2”. Put your finished block on top of the 12 1/2 square ruler is what Janice suggests. It is worth it to buy this ruler if you are doing the boom. Go to Joann’s with a 50% off coupon.
I'm going to interrupt and put my 2 cents in here.... thats backward you need to put the square on top of the fabric otherwise you may not get an accurate measurement.