Hi...my method is pin, string or dental floss, and pencil or chalk marker.
I lay my fabric on my cardboard mat stick & straight pins around it to keep it from moving - stick a large straight pin the middle..tie the string (dental floss works well) around the pin....measure the length (radius of the circle) plus enough to tie and tie a pencil to the other end...hold the center pin firmly - pull gently on the string to keep it taut - go around in a circle. I've made up to 30" diameter this way.
Not very techie - but it works.....we're very "old school" (and just plain old) around this house !!!
Ha...Tartan was answering while i was typing!!!!! No offense with the "old" comments Tartan...that was only for myself !!