Next, we're going to use some quick math to create a circle
template for the bottom of the bag. Don't worry, it's easy! If
you're making a bag the same size as mine, you can just use my measurements
exactly, but if you want to modify the size of your bag at all,
here's how to get there...
Measure the length of fabric (13 inches), minus the 0.25 inch
seam allowance on both sides, which equals 12.5 inches. Then divide
your length by 3.14, which in this case equals 3.98 inches. I have
rounded that up to 4 inches. Finally, we're going to add a 0.25 inch
seam allowance around my circle. So I want a circle with a 4.5 inch
diameter. Dividing that in half gives me a 2.25 inch radius.