Life size

[mcgivney]

NewBee here.
I scan a ruler to a jpeg at 300 dpi, grayscale and call it myscan.jpg
I open myscan.jpg in Gimp 2.8.18
I change the zoom to 100%
The ruler in now much bigger than life size.
Using the measure tool, I draw a straight line between two points 50 mm apart on the ruler, Gimp tells me the line is also 50 mm, which is correct.
To make the ruler life size on the screen, I need to zoom-out to 45%. Again I draw the line with the measure tool and again it is 50 mm.
My question: why is the ruler not life size at 100% zoom? Why is 100% zoom much bigger than life size on the screen?
Thanks,
Jim

[ythgilb]

.
This forum is DEAD !

Folk have moved to
https://www.gimp-forum.net
Questions can be answered there