Today’s Question: I’ve bought into the 72 ppi [pixel per inch] misinformation for 20 years. Curious if it was EVER true. Since we can’t know what displays our clients will use, what should we do for a photo in an email or on a website? Should I just pick a number around 200 [for ppi resolution] and hope for the best?
Tim’s Quick Answer: The pixel per inch (ppi) resolution is irrelevant for images that will only be displayed digitally, as only the actual pixel dimensions really matter in that context. The 72 ppi value was actually accurate for the original Macintosh 9-inch display, which featured a resolution of 512×342 pixels (not a lot of pixels!).
More Detail: Today’s question is a follow-up to my answer from the December 12th newsletter where I addressed the general topic of pixel per inch (ppi) resolution for digital images. As noted in that answer, the ppi resolution is only a factor when printing. Even then, it is really there for convenience. Referencing a ppi resolution enables you to describe a print size in more familiar terms. Instead of saying that you need to resize an image to 2,400 pixels by 3,000 pixels, you can say that you need to resize the image to 8-inches by 10-inches at 300 pixels per inch.
While the ppi resolution really doesn’t mean anything for images intended to be displayed digitally, that isn’t to say you should necessarily completely ignore this value when resizing an image for digital sharing.
I most certainly don’t suggest setting the image to a resolution of 72 ppi, since that doesn’t align with any method for sharing images today. I also don’t recommend the 96 ppi value that had similarly become commonly used with the (false) notion that Windows computers used a display resolution of 96 ppi while Macintosh computers used 72 ppi.
I think the best default value for the ppi resolution for images that will be shared digitally is a resolution that would be suitable for printing. That way if the image does get printed by a client, for example, it would already be configured at a reasonable setting.
While output resolution varies depending on printing equipment and other factors, a good general value is 300 ppi. So, for images that are going to be shared digitally, the ppi resolution doesn’t really matter at all. But you may as well set it to something like 300 ppi by default, since that is a commonly used resolution setting when an image is printed.