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

To access a Radial dimension you can:
select Radial under the Dimension Menu,
OR
type dimrad or dra at a Command: prompt,
OR
click on the Radial
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: 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.