"The" pixel does not exist as an SI approved measurement unit.
| Now the issue is I was checking online and found a few 'convert from pixels to millimetres' and they come up with 92mm x 132mm. |
Can you give the URL of such a website? 'Cause I'm gonna send them a mail: "You, Sir, are an Idiot to offer this." 
A simple experiment will prove my point. Please indulge me: draw a rectangle in Photoshop 350 pixels wide by 500 pixels deep. Make sure to have the zoom set at 100%. Now take a ruler and measure the size of this rectangle is in millimeters.
Does that give you a calculation for "pixels to millimeters"? Of course it does, but only for your own screen. What you are measuring then is your local screen dpi. I could do the same and get another "real world size" because I'm positively sure my screen has a different number of dpi than yours, and the screen of the web "master" of "convertFromPixelsToMillimeters.com" will have yet another number of dpi.
It's a shame you are most likely using a modern LED or LCD screen, because with an ol' analog monitors I have an even better example: by fiddling with the dials on the front, you can expand or contract the electron beam to make your local view bigger or smaller. But the number of pixels stays the same!
Which one is correct? And why is Photoshop saying one thing and the online converter another? |
Photoshop is correct. It is the only software in which you tie dimensions in pixels to a number of pixels per measurement unit -- and at that point, it's simple math.
If you are asked to design an ad of 350 x 500 pixels, don't worry about anything of the above. Those are the values to enter in Photoshop -- "dpi" values are inconsequential for the web.
(You would need to worry if you got contradicting instructions such as "3.5 x 5 inch, oh yeah and it's for the web by the way".)
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