Are you looking for an answer other than "The perimeter of a circle is given by $2\pi r$"? Because most common shapes already have formulas for their perimeter. (And for arbitrary parameterized shapes you can just use a line integral, which is basically another fomula)

Hello Moshi, thank you, sadly I don't know what are the Greek letters there and can't copy-paste what you wrote there to learn about it because it's an image. I don't know what a "formula for a perimeter" mean besides in computing when I do for example border: 2px solid black so I know that the perimeter would be 2px large.

deleted user When you do border: 2px solid black, you make it 2px thick. The length of the perimeter would not be 2px.

The Greek letter there is pi, https://en.wikipedia.org/wiki/Pi

Moshi‭i, thanks, I understand any circle to be the border of the disk, so if that border got 2 px thickness, won't it be correct that the circle's perimeter is 2px thick?

deleted user Mathematical perimeters do not have a concept of width, since they are one dimensional. What you're describing is an annulus.