Background Information: (As far as I know)

"PPI" stands for "Pixels Per Inch", and refers to how many pixels there are, predictably, per inch in any given monitor.

"DPI" stands for "Dots Per Inch", and generally refers to how many ink dots a printer will place per inch on a piece of paper.

The more there are of either, the better the resulting image usually is, to a point. PPI and DPI don't need to match for you to size images properly. Your graphics program should take care of the conversion for you. For some reason, DPI has also come to equal PPI for many people. "High DPI" refers to monitors with an many more pixels densely packed into a inch on the screen than normal.

It is important to account for PPI/DPI when sizing anything on the display. Two monitors can make an image look vastly different depending on their PPI.

A 1080p (1920x1080) monitor with 100 pixels per inch will be 10.8 (aprox. 10 and 13/16) inches tall.
A 1080p (1920x1080) monitor with 200 pixels per inch will be 5.4 (aprox. 5 and 6/16) inches tall.

-----

Expectation:

Using the "Use same aspect as pixels" feature should resize how the image is displayed so that one inch on the canvas equals one inch of a ruler held up to the screen. That is how the "dot for dot" option in GIMP works. The feature is incredibly useful when it works.

-----

What Happens:

Toggling the button will make an image on the canvas switch from larger than, to smaller than, an inch on a ruler. It isn't calibrated correctly.

-----

Comments:

I think this happens because Krita doesn't seem to detect my monitor PPI correctly. It also lacks an option for specifying your monitor's PPI. My monitor has 117 PPI, and judging off of some rough math, Krita seems to think my monitor has 96 PPI. This means that things are either too big or about 20% too small, and I have to manually adjust the zoom if I want to see an image at its "real" size.

This means that I cannot reliably gauge what size something will be when I print it, among other things.

-----

Solution:

Krita could work like GIMP and simply have an option to turn off automatic PPI detection so that users could enter the proper PPI for their monitor. Then no developers would need to figure out complicated fixes that may differ from computer to computer and OS to OS. Just add a small text box where I can type in "117" instead of "96" so that Krita will scale images properly.

All I want is an inch on my monitor to equal an inch in real life.

-----

More Info:
https://en.wikipedia.org/wiki/Dots_per_inch
https://en.wikipedia.org/wiki/Pixel_density