I use circles all the time in my patternmaking as a seamstress. I had a lot I had cut from tackboard, but I wanted something more permanent.
I labeled them with the radius and the circumference, as well as half and quarter of the circumference so I don’t have to do any math. I use the ¼ to round out corners on upholstery.
How? You put the circle in the corner and mark where the quarter lines land. You measure between the marks on the strait edges, then add the amount of the ¼ of the circle.
I also marked how long you would cut a band (using a ½” seam) for making bags.
ALSO....the Lesson (there’s always a lesson, it seems) be sure when doing your math to put on the circles, you don’t use the diameter instead of the radius. OOPS! All of my circles have the wrong math......time to make new circles. Good thing they were cheap!