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I was told that when I make a screenshot (for example, using Shift-Command-4 > Drag), it is saved with system PPI.

But the spec for my Mac says that its resolution is 254, not 144.

MacBook Pro (14-inch, 2021) - Technical Specifications: [...] 14.2-inch (diagonal) Liquid Retina XDR display; 3024-by-1964 native resolution at 254 pixels per inch - https://support.apple.com/111902

So what does it all mean?

  • Does it mean that PPI of screenshots and system PPI are different (144 and 254 respectively)?
  • Or does it mean that "PPI" and "resolution" are different things?
    • And if yes, then why Mac uses 144 PPI and not, for example, 216, 288, or maybe 300 for screenshots?
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  • When you create a screencapture, what format are you saving it as, PNG, JPG, GIF, TIFF, PDF? If PNG, then the resolution should be metric - 56.69 ppc (pixels per centimeter) and the 144 refers to the resolution bytes, not pixels.
    – Allan
    Commented May 6 at 22:39

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Your screen is 3024x1964 pixels, so all full screenshots have an image size 3024x1964 pixels. It so happens that on your physical screen this is 254 pixels per inch (PPI).

On my screen the full screenshot has an image size of 5120x2880 pixels and with my physical screen this is 220 pixels per inch.

If you were to send me your image, my computer would display all 3024x1964 pixels but at the screen resolution of my screen (220 PPI). So it will be physically larger than on your computer.

For display on a computer screen, the image has a size in pixels and displays at the ppi of whatever physical screen is being used. Pixels per inch on screen is not a property of the image - rather of the display.

But what has been included in the image metadata is how the image will print. Scaling for printing is measured by dots per inch (DPI) on paper. All screenshots created by macOS 14 are labelled with a printing resolution of 144 DPI. Preview's Inspector describes this as "Image DPI: 144 pixels/inch". This is pixel per inch for printing - not computer display.

This means that all printers will print the image at the same physical size using a resolution of 144 dots/inch. So your printer will print a picture which is 21" by 13.6". And my printer will print it at the same size.

The DPI can be adjusted in Preview's Tools > Adjust Size. Here is an adjustment of my full screenshot so that it will fit easily on an A4 printer but without changing the pixels in the image file.

enter image description here

The size on paper can also be adjusted in the print dialog. This is often set to automatically "fit on paper".

What does it all mean? There are two different resolutions:

  1. The resolution of your screen (254 PPI), and
  2. The default resolution to be used when printing (DPI of 144 pixels/inch) which is an item in the image metadata.

Why 144?

Typesetters measure font size in points. 72 points is 1 inch. Early Macs had a screen resolution of 72 pixels/inch (one reason they were liked for desktop publishing) so it was natural for images to have a DPI of 72 pixels/inch. For a long time this remained unchanged even though screen resolution had changed. My understanding is that Apple doubled the DPI to 144 pixels/inch for Macs with "Retina" screens.

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  • Excellent explanation, thanks a lot!
    – user480875
    Commented May 6 at 7:04
  • 72dpi was always the default resolution for screen, not for print. The first Apple Macs had 72dpi screens, and so there was initially a relationship that "1 pixel on screen = 1 point on the paper", but the first Apple LaserWriter in 1985 had a resolution of 300dpi. Printers (e.g. shops) will always ask for bitmap images at 300dpi; and if you try printing an image at 72dpi, it will look awful. 72dpi was never "natural" for printing.
    – benwiggy
    Commented May 6 at 7:40
  • @benwiggy I agree the 72dpi or 144dpi have never made sense since the early days. Anyone sending images to print shops needs to get a complete workflow established.
    – Gilby
    Commented May 6 at 9:28
  • Images for web were traditionally specced at 72dpi; though this went up to 96dpi because of some requirement for Windows. My point is that your suggestion that 72dpi comes from printing should be altered.
    – benwiggy
    Commented May 6 at 10:16

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