File size/dimensions

New Here ,
May 29, 2022 May 29, 2022

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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. 

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Adobe Community Professional ,
May 29, 2022 May 29, 2022

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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

 

Dave

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New Here ,
May 29, 2022 May 29, 2022

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So what should I do next time? Just work on pixels? 

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Adobe Community Professional ,
May 29, 2022 May 29, 2022

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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.

 

Dave

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New Here ,
May 29, 2022 May 29, 2022

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I did it now and after making in cm I changed to pixels and it still is off my 1 mm 

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