Right mouse click over the dimension and select "display as diameter". You may need the dimension tool selected as the time. I didn't double check before answering.
Thank you Don,I tried that before but just didn't see it in the list then. Just clicked on it, right click and find Display as Diameter.
You might have missed it the first time because it's not directly in the right-click menu, but in the flyout menu under Display Options.
Another method is to go to the Leaders tab on the dimensions Property Manager and select the appropriate icon.
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