A RADIAL dimension allows you to place a radius dimension on an arc.

To access a Radius dimension you can:
select Radius under the Dimension Menu,
OR
type dimrad or dra at a Command: prompt,
OR
click on the Radius
button
on the Dimensioning
toolbar.
Prompt: Command: Type dimrad and Enter.
Prompt: Select arc or circle: Click on the arc you wish to dimension.
Prompt: Dimension text = dimension value. AutoCAD indicates the numeric value of the radius of the feature you selected.
Prompt: Specify dimension line location [Mtext/Text/Angle]: Click the Left mouse button on the screen to locate the position for radius text.
*NOTE: If you wish
to add text to the dimension, select the Mtext option under the last
prompt.
Return to the
AutoCAD Command List.