Pixel density calculator
Pixel density is the diagonal pixel count divided by the physical diagonal. A 6.1-inch phone at 1170×2532 works out at 457 PPI. Apple markets the same panel as 460 — well above the roughly 300 PPI at which individual pixels stop being resolvable at typical phone viewing distance.
How to work out pixel density
Phone densities have far outrun what is visually necessary, and the reason is not sharpness. Very high density gives room for subpixel arrangements like PenTile that trade real subpixels for panel efficiency, and it allows clean integer scaling of interface layouts across a range of physical sizes. A 460 PPI phone is not visibly sharper than a 380 PPI one at arm length; it is easier to build a consistent interface on.
Questions
About 300 PPI at 30 cm, which is why that figure gets quoted for phones. It falls with distance.
Not quite. PPI describes a display; DPI describes printing. The arithmetic is similar and the terms get used interchangeably.
Partly marketing, partly to support subpixel layouts and clean interface scaling. It is not about visible sharpness.
Yes, somewhat — more pixels to drive and light. It is one reason some phones run below native resolution by default.
The physical distance between pixel centres, in millimetres. It is just the inverse of density.