100% vs Print Size vs Actual size : something stays unclear

davescm got the fundamentals right. Some important additional notes:

Actual Size works for print only, where the unit of measure is inches. Actual Size does not work and must not be used for previewing the real world viewing size for documents where pixels are the unit of measure (mobile, web, or video). I think the command should have been named more precisely since print is probably not a majority delivery method for images these days.

Actual Size requires that the document ppi value (in Image > Image Size) matches the ppi at which the document will be printed. If you have two documents with the same pixel dimensions (such was 900 𝗑 600 px), with one set to 300 ppi and the other set to 240 ppi, and you set both to Actual Size magnification, the rulers will display exactly the same correct real world size in both documents, but the 240 ppi document will appear larger. The one that is the correct size is the one matching the final print resolution.

And now the big one…

Where the new Actual Size magnification makes the most difference over Print Size is on displays set to Retina/HiDPI (2x scale factor) resolutions. Actual Size usually gets it right (one inch on screen ruler matches real world ruler) regardless of the display scale factor, but the Print Size method will fail unless you account for both the UI resolution (the Scaled setting in Displays) and the Retina scale factor. For example, my MacBook Pro display hardware resolution is 227 ppi, but if you enter that for Print Size ppi, it will be wrong for 3 of the 4 Scaled options in the Displays system preference. What you need to enter in Print Size is the UI (not hardware) resolution times the Retina scale factor. For example, when the selected Scaled setting is Default (“Looks Like 1440 𝗑 900”), the correct ppi value for Print Size is 256, because that is 2x (Retina scale factor) the UI resolution (128 ppi). It turns out that the InDesign script in this thread is a nice quick way to get the correct 1x-equivalent ppi resolution for the Print Size method, because it gives you the UI resolution, the one that changes depending on the Scaled setting in Displays. (We didn’t have to think about this on 1x displays because the UI ppi almost always matched the hardware ppi.)

*The reason the basic Print Size method still works for one of Scaled options is because that one (“Looks like 1280 𝗑 800”) is exactly half the 2560 𝗑 1600 hardware resolution, so 2x that equals 2560 𝗑 1600, same result as doing Print Size for a 2560 𝗑 1600 non-Retina display.

Knowing the calculation for Retina (Mac) and HiDPI (Windows) displays is important because they are becoming so much more common. All Mac displays are now Retina.

Mine behaves pretty much the same as your Windows machine, except mine seems to be using 108ppi in Actual Size, when the correct value, and what I entered in settings, of 109ppi. So it doesn't appear to be a fixed 100ppi, but rather a variable number that is just off. I added my report to your thread. Not holding my breath for it to be addressed though.

Sorry to bump an old thread and thanks for the info. Where does "acutal" size get the monitor resolution and physical size information? I set my Screen Resolution based on dividing my display's horizontal pixels by the measured width and now it is spot on when I use Print Size. Actual Size is a couple percent too small though. Kind of an esoteric question, I guess, since Print Size works perfectly well, but I am curious where PS gets the pixels/inch number from and why it's wrong. Thanks!