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I scanned an 210X297 mm page and the scanner gave me a 899.58x1238.25 mm image. (not sure why).

I am trying to resize the image to be 220x310 mm. The bit that confuses me is that is the measurement of images in the computer rely on pixels? If so, how much is that in mm?

Since the size difference is so big, obviously, my photo will be pixelated.

  • Pixels do not have a size they are differently sized on every monitor! – joojaa Mar 28 '17 at 15:30
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The image is made of pixels (obviously always an integer number of them), and the print size is obtained by multiplying the size in pixels by the print definition in pixels-per-inch or pixels-per-millimetre. In other words there is no true physical size.

Now it all depends of the size in pixels (which is something you don't mention). You can set the print size to anything you want using Image>Print size and changing the print resolution. The image will appear pixelated if the print definition is below 100PPI. How acceptable this is depends on the image content. To give you an idea of print definition:

  • Last consumer-grade CRT displays: 72-80 PPI
  • Current LCD: 100-120PPI
  • High definition LCD: 200 PPI
  • Draft paper printing: 150 PPI
  • Standard office printing: 300PPI
  • High quality office printing: 600PPI

Note that your scanned image has not got the right aspect ratio for an A4 (1238.25/899.58 is 1.38 when it should be 1.41) so you may have to crop it a bit.

  • This was a clear and informative reply, It helped me a lot! thanks! – Angelo Charl Mar 30 '17 at 12:35
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I suspect the problem you have is because the resolution of the scanned image is not the same as the resolution setting in GIMP. When you open an image in GIMP, if you do not set the resolution exactly the same as the scan, then the size will show as different.

It's easy enough to fix. Take a note of the resolution you scanned the image at. You might need to look through your scanner settings to find out what the resolution is set at. Then in GIMP, click Image > Print Size. Enter the resolution you scanned the image at. This should fix the size problem.

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    Right: 210 x 300/ 72 = 875. So, probably the item was scanned at 300ppi, then saved as a jpeg, which does not always store the dpi flag (it is optional and not all software will even query nor honor the flag). 72ppi is often the "default presumption" in software when it is unaware of the intended print resolution. Possibly the person entered "300" in the resolution field when scanning, even though the measurements are in mm (or something, the thing is the math works out). – Yorik Mar 28 '17 at 17:56

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