Koala Tools - Mini Ring Ruler

The Ring Ruler Mini™ is a compact size-adjustable circle ruler. Simply click the zero point end into your desired size and it holds in place for easy measuring, marking or forming. The ergonomic benefit of be able to have your pencil or pen in your hand for marking means that you have greater control of the line density and pressure. Expanding and contracting the circle is as easy as pulling the ruler to form your desired size.
 The Ring Ruler Mini also provides useful circular metrics like diameter and can also be used as a 360 degree protractor.