How To Find the Radius of a Circle From Just 2 Measurements

Added: 2026-05-17

[00:00:01]The customer wants a new retaining wall done. This one's old, falling apart, and the blocks are overgrown with moss.

[00:00:09]She's thinking bigger blocks, the same radius, and a metal handrail on top. So, I need to measure the radius so I can get the materials here. To do that, I'm going to take my 4ft level and place it up against the blocking like this. And then I measure the gap at the widest point, ensuring that both sides read the same when I do this. Then you would take a few more measurements in order to get your average. And again, this is just a rough estimate so that I can get materials coming. So, I'm going to show you how accurate this actually is. 12 in.

[00:00:40]3 and 1/4 gap.

[00:00:43]Now, we do a little math. This is your cord. Square that. This gap is your height. Multiply that by 8. Divide your cord into your height and then add half of your height. Our cord is 12 in long.

[00:00:56]12^2 is 144.

[00:00:59]Our height is 3 and a/4. Times that by 8, get 26. Half of 3 and 1/4 is 1 and 5/8.

[00:01:08]144 / 26, we get 5.538.

[00:01:12]Add to that 1 and 5/8, we get 7.163.

[00:01:17]Times that by 2 for our diameter, we get 14 and 516.

[00:01:22]Now I'll measure straight across and see what we have.

[00:01:26]14 and 516. This works really well when you can't measure across the top and you can only get to it from the side. That's it for this one. I'll see you on the next.