Mac Stamp Measuring Tool Wanted

So I posted in another thread trying to determine what others use to measure scans / details of stamps. No response so I thought id throw this up. Im on a Mac and would like an accurate way to easily measure stamp details - line thickness, etc. I saw this, calipers from Iconico, in a search here but it seems only available in Windows - and more than what I wanted to spend.

What do you all recommend and why?

I use Fiji. It's designed for scientific image analysis (microscopes, medical, etc...) so it has all the stuff I need -- and a ton more I don't need. I like the way it is easy to extend and automate -- I have developed a number of Fiji/ImageJ macros for stamp related stuff over the years.

https://imagej.net/software/fiji/?Downloads

Most (all?) image programs have a method to measure. I use Pixelmator (Mac only). Here is screenshot. Ruler is set to millimeters.

tks both - ill check them out