# How can I calculate the value for a corner radius?

I'm not asking how to solve what a corner radius is on a piece, rather how to calculate what corner radius I should use on a piece.

I need to make a (negative) template to follow with my router. In order to do so, I need to use a hole saw to drill out a curved corner. I'm having trouble calculating what the corner radius would be such that I can determine what size hole saw to use.

The picture probably explains better than my words, but essentially I want to round off the corners of a rectangle with the curve starting 3/4" away from the corner. I don't know if this is more math but it's something I've run into previously while woodworking.

How would I determine both the size of the hole saw to use as well as the distance I should center it from the corner (r)?

• I'm not exactly sure I understand your question. I think the most clear way to describe your problem would be to add an image of the desired final result. If you also post the dimensions of your router and the bit you'd like to use it should be clear on how to design a template.
– null
Oct 27 '15 at 19:57

## 2 Answers

Your diagram I think doesn't actually clarify the situation.

I want to round off the corners of a rectangle with the curve starting 3/4" away from the corner

If I'm reading this correctly 3/4" is your radius, so double that is the diameter of the bit you need to use. I don't think it's any more complicated than that.

To place the centre point in relation to the corner, you can do that just by marking outwards in both directions 3/4" and using a try square to cross the lines. Alternatively if you want to mark along the 45°, using Pythagoras it'll be 1.06" (or just a hair under 27mm) from the corner.

• I thought the same when looking at the diagram
– null
Oct 27 '15 at 19:09
• Well, now I feel sort of silly. Thank you! Oct 28 '15 at 8:44

If you want the curve to start 3/4" away from the corner the radius of your curve is going to be 3/4". So, to make that curve with a drill bit, double the radius to get the diameter of the bit... 1-1/2" bit. It would be centered 3/4" away from each side.