I manufacture Arc Master Radius Gauges. The IRG 3600 measures the radius of an arc from the Inside while the ORG 3600 measures arcs from the Outside. These gauges have a measuring range from 6"(150mm) to 144"(3600mm) in Metric and Imperial. They require 12" to 13" of arc to obtain a measurement. An attachment for the IRG 3600 & ORG 3600 is the Variable Arc. This unique tool, when mounted on the radius gauges, is designed to form the actual arc the radius gauge is set to. I also manufacture a set of smaller radius gauges. The IRG 750 and the ORG 750. These measure arc from 2.5"(64mm) to 30"(750mm) from the inside and outside respectively. They require only 5"(128mm) of arc to obtain a measurement.