I use PS 2020 and 2021 in the latest versions on an iMac. The following "problem" I have in both versions (2020 and 21). When I create a new image file, as an example 400 mm wide, 200 mm high at a resolution of 300 dpi, it is 399.97 x 199.98 mm in size when it is controlled by image size. They are, of course, only one hundredths, but it would still be important that the measure is accurate. Did I set something wrong in the presets?
Digital images do not have physical dimensions, they only have pixel dimensions.
The ppi (pixels per inch) value is used to calculate physical dimensions, pixel dimensions divided by ppi value = physical dimensions in inches.
Photoshop will do the math for you, and let you create an image with physical dimensions.
The image still does not have physical dimensions, but when printed, the printer driver will calculate physical dimensions using the ppi.
Your image is 4724 x 2362 pixels, divide this by 300, and you get 15.747 x 7.873 inches.
Multiply these numbers by 25.4, and you get 399.9738 x 199.9742 millimeters.
This is the closest you can get to 400 x 200 mm, because you can't have fractions of a pixel, they have to be whole.
If you increase the pixel dimensions to 4725 x 2363, you get 400.05 x 200.06733 mm.
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