You can't - until you know where that picture is going to be used. It depends on DPI (dots per inch) of the output device.

Is it going to be displayed on a monitor? Then it might be 100 DPI or 75 DPI (those two values are usualy used for the sizing of fonts) or if the user has played with the monitor resolution, it might be some random number. (But generaly in that range).

If the image is going to be printed, that is usualy a much higher DPI than the monitor - 300 and up. High quality linotype printers generaly start at 2500 DPI.

In reply to Re: Conversion Modules from Pixels to Millimeters by matija
in thread Conversion Modules from Pixels to Millimeters by Anonymous Monk

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