How can I tell the true scan resolution (PPI) of an image file?
Asked 6/12/2026
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2 answers
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I was given scanned image files on a USB drive. The vendor says the scans are "360 dpi," but my computer’s file info shows 96 ppi. I’m confused about which value is meaningful.
For example, one image is 2706 pixels wide, and Finder reports a resolution of 96. I’ve also been told that the monitor affects this number, but that doesn’t seem right.
How can I determine the actual scan resolution of the file? Does the 96 ppi metadata mean the scan was not done at 360 ppi?
Originally by Stumped. Source · Licensed CC BY-SA 4.0
Stumped
1mo ago
2 Answers
11
We have a canonical question on this site about the meaning of the "DPI" field in digital image metadata; the short version is that it is generally meaningless.
However, you can work this out. Assuming that the objects in the image are perfectly standard sized Penny Blacks which have a size of 7/8" × 3/4", the image is (9 × 3/4") = 27/4" wide. Add a little bit for the spacing between the stamps and let's call it 7"; you can do a more accurate assessment of the spacing if you want to. You know how wide the image is in pixels (look at the original, not the one uploaded here as Stack Exchange shrinks images), so just divide that number of pixels by 7 and you have the PPI.
With regards to the Penny Red image, what you show is perfectly consistent with it being at about 360 ppi. Ignore the metadata and use your brain: 3.55" at 96 ppi means the image is 340 pixels high. At 360 ppi, that makes it about 0.95". Take off a little bit for the padding, and you're at 7/8", the height of a Penny Red.
Originally by Philip Kendall. Source · Licensed CC BY-SA 4.0
Philip Kendall
1mo ago
0
Generated from our catalog & community — verify before relying on it.
The 96 ppi shown in Finder is usually just metadata and often does not tell you the real scan resolution. It does not mean your monitor changed the file, and it does not by itself prove the scan was done incorrectly.
What matters is the pixel dimensions of the file compared with the physical size of the original item scanned.
To estimate the true scan resolution:
- Find the image width in pixels.
- Measure the original subject’s actual width in inches.
- Divide pixels by inches.
Example from the answers: if the file is 2706 px wide and the original subject is about 6.75 inches wide, then: 2706 ÷ 6.75 ≈ 400 ppi
So a file can display 96 ppi in metadata and still contain enough pixels to correspond to a 360–400 ppi scan of the original.
In short: ignore the Finder ppi field for this purpose. Use pixel dimensions and the original object’s size to determine the effective scan resolution.
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