Hi, I have a problem with the size of my files. When I create them I choose for exaple 20x20 cm and it seems like it will stay that way, but every time I check and open the file in PS it changes it's size by few mm and unfortunately it's bugging me a lot because my professors say it can't happen. What can I do? I feel like I tried everything and also I can't find reason for it to happen.
Photoshop works in pixels. There are no half pixels or any other fractions of a pixel, only whole pixels.
20cm at 300pixels per inch requires 20/2.54 *300 = 2362.204 pixels. As said above you can't have part of a pixel so it has to be rounded up or rounded down.
Rounding down to 2362 pixels at 300ppi gives 19.99cm.
Rounding up to 2363 pixels at 300ppi gives 20.006cm
Work in entirely in pixels or use a resolution (ppi or ppcm) value that gives a whole pixel division for your image dimension. For example working at 118 ppcm instead of 300ppi gives exactly 2360 pixels for a 20cm wide document.
Working in a mix of cm, inches(ppi) and pixels is always going to lead to rounding errors. Working with cm , ppcm and pixels can avoid those.
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