First You would use a ruler and measure the horizontal length of your screen. Then look at your resolution setting. In my case if my resolution was 1280 X 800 and my screen was 10 inches
, that would be a screen resolution of 128 ppi. 1280÷10.
I then go into Photoshop's preferences, Units and Rulers, and under document presets, I replace 72 with 128. Of course, you can do all in cm. Click OK to save and close.
Now open your document, turn on the rulers. From the View menu, select Print Size, and your screen rulers should match a physical ruler.
If you are working with a paint program that does not have this option, use the zoom tool to match the onscreen rulers against a physical ruler, and make a note of the zoom percentage.
Then you can enter that value in your program's zoom box to see your work in cm.
Thank you so much for the help:-)