I want to match the existing radius on my stairs trim, but I'm unsure what size rounding over bit I need to get. I measured the arc of the radius the best I could with a flexible tape measure and it's about 9/16". I did the math and that equates to a 0.6" radius, which is close to 5/8". Perhaps I'm not measuring correctly. Is there a common radius for 3/4" board?
There are a brazillion router bit roundover sizes. From somewhere with a gentle return policy, buy the one you think you need plus sizes above and below.
Or if you're going to a brick+mortar, you could create a profile template (stiff cardboard) of your stair and compare it with what's on the shelf.
First, setup a lamp shinning on the rounded edge.
Get two rulers, place one perpendicular the vertical side of the stair and the other perpendicular the horizontal side. Then line rulers up flush to each other, say at the 4” mark, you should be able to get a measurement of where the gap starts (from both directions). That’s the radius!
The idea is you are making a small carpenters square (image below), so you see the radius of the circle. The light shinning from behind should make it easier to see where the gap between a ruler and the stair starts.
Well, what I would do is grab my collection of router bits and hold a likely suspect next to the step to see if that bit matches.
If it was routed in the U.S. by a U.S. craftsman, it is probably one of these four bits which can be obtained for $20 (for the set).