When making Free Body Diagrams for my engineering classes, I found I had to make lots of circles. Having the artistic ability of a walrus, I was often making my wheels into potatoes, asteroids, and amoebas. While compases are nice for larger diagrams, they aren't really good for smaller circles (and it puts a small hole in your nice clean paper).
I decided to make this handy little guide (which is about the size of a credit card) to help me make circles for Free Body Diagrams (or whatever else I need a perfect circle for), and then put it in my wallet for later.
The guide features 2 sides, one with metric diameters ( in 5mm, 10mm, 15mm and 20mm)
and one with English Units (in 1/4in, 1/2in, 3/4in, and 1in), so no matter what you need you can make a nice circle.