[How can I measure the length of a line?]

Thank you! I knew there was something easy that I was overlooking.

[How can I measure the length of a line?]

Hello, and thank you in advance for any help. I am wanting to use paint.net to draw lines on an image and then measure the length of those lines. I have the ruler turned on and I see that it is showing pixel coordinates, but I need actual length (for example, comparing the length of a right femur to a left femur). The work that I am trying to do is based purely off of comparing the percentages of right vs. left, so the units don't matter. I've never used PDN before, so I'm sure there is probably something simple that I have overlooked.