Arc Length

How do I know the length arc or diameter like this picture in layout? https://twitter.com/messages/media/1205172691021549572

you do in SU…

I think the OP probably meant how would he show it rather than know it.

I don’t think there is a native way of showing it. I would like to add to this request an ability to show a radius in a more conventional way rather than having to show an offset dimension line from centrepoint to arc/circle. Usually it means having something like an arrow with the dimension preceded by the letter R.