PPI needs two kinds of measurement: the native pixel grid and the physical size of the visible panel. A 2560×1440 image has the same pixel count on every device, but its density changes when that grid is spread across a different diagonal.
Find the pixel diagonal
Apply the Pythagorean theorem: √(width² + height²). For 2560×1440 the result is approximately 2,937.209560 pixels. Width and height must be the native active pixel dimensions, not a scaled desktop mode.
Divide by the visible diagonal
PPI = pixel diagonal ÷ diagonal in inches. A 27-inch 2560×1440 panel is approximately 2937.209560÷27 = 108.785539 PPI. If diagonal is measured in centimeters, divide it by 2.54 first.
Keep the measurement boundary clear
A nominal diagonal may be rounded and the visible active area may differ from a case dimension. PPI is geometry, not a rating of sharpness, eyesight, viewing distance, image quality or suitability for a specialized task.