# Deriving radii for round corners

A lot of 3D work I do involves working with round corners. In this case, I was designing a case to be fitted onto an appliance with a round corner.

The item was essentially rectangular except with corners rounded. While I could figure out the dimensions of the item quite easily with a calliper, I couldn’t figure out the radius of the corners (which I need in order to tell openSCAD).

I tried using all sorts of geometry tricks to no avail until I finally figured it out by using a tape measure.

https://cdn.mediacru.sh/jprbcB45O_aD.svg

What I ended up doing to find the radius of the corner was to calculate the perimeter of the item if it were rectangular (2 x width + 2 x height) and subtract the actual perimeter of the device.

That leaves the perimeter of the ‘corner’ edges which there are 8 (two on each corner). Dividing the remaining perimeter by 8 yields the radius of the border.

Of course, you’ll want to be super careful on how you measure and make sure you’re getting an accurate measurement! Furthermore, this will only work for objects that have corners that have the same radius.