Abstract: A tool for determining the radius, e.g., of a construction or other surface, such as a doorway arch or the like, has three arms. Each arm has a free end and defines an elongated slot through which a pivot pin is mounted. In one embodiment, arm free ends are engaged with an arch or other surface, and the arms and pivot pin are relatively adjusted until the same dimension is indicated on each arm. In another embodiment, the slots and the pivot pin define cooperatively interengaging teeth, and rotation of the pivot pin moves the arms until the free ends of all the arms engage the surface to indicate the surface radius. The radius tool may also have the form of a flexible arms wound from a spool.