PPI calculator
A 27-inch 1440p monitor at 109 PPI looks sharp at arm length and coarse if you lean in. A phone at 460 PPI looks sharp at 30 cm and would look coarse held to your nose. What actually determines whether pixels are visible is angular resolution: the eye resolves about one arcminute, so the required density falls linearly with viewing distance. A billboard at 10 PPI is perfectly sharp from the road.
Pixel density is the diagonal pixel count divided by the diagonal in inches. A 27-inch 2560×1440 monitor works out at 108.79 PPI, with a dot pitch of 0.2335 mm and a physical panel 59.7 cm wide.
How to calculate PPI
Around 110 PPI is the comfortable zone for a desktop monitor at normal viewing distance, which is why 24-inch 1080p and 27-inch 1440p both feel similar and why 27-inch 1080p at 82 PPI looks noticeably soft. Above about 150 PPI, operating systems generally need fractional scaling to keep interface elements a usable size, and how well that is handled varies considerably between platforms — which is a practical argument for choosing a density that lands near a clean 100% or 200% rather than at 150.
Questions
Around 110 for a desktop at arm length. 24-inch 1080p and 27-inch 1440p both land close to it.
The distance between pixel centres; 25.4 divided by PPI. Lower is finer.
Whatever exceeds what the eye can resolve at the viewing distance. Roughly 300 PPI for a phone, 220 for a laptop, 160 for a desktop.
Only up to the point the eye can resolve. Beyond that it costs performance and may force awkward scaling.
Display settings on any operating system, or the specification sheet for the model.