A radius gauge or fillet gauge is a tool used to measure the radius of a curve. These tools are used for welding and light engineering work. They are typically designed to measure a radius of less than 150 degrees. There are several different styles of gauges, depending on the application and the manufacturer. A quality tool will measure both concave and convex sections.
Hold the gauge perpendicular to the curved object for which you need to know the radius.
Press the centre of the gauge to the object and adjust the gauge so that the two arms touch the curve without moving the centre point from the surface.
Read the gauge's degree indicator located near the handle.