You can measure the radius of a circle or arc in the viewport. The measurable drawing elements can be any of the following: view and added geometry, center and symmetry lines, sketches, and the sheet frame.
To measure a radius
1. On the 3D Access — Annotation tab, in the Utilities group, click the arrow next to Between Points.
2. Click Radius. The Measure dialog box opens.
3. Click a circular element in the viewport to measure. The element's radius is displayed in the Radius Output box.
4. Continue clicking elements, or click or to complete the command.
You can click OK to use the output from Radius as input for an interrupted operation.
Between Points.
Radius. The Measure dialog box opens.
or 
to complete the command.