PPI Calculator
Calculates screen pixel density from resolution and diagonal, and print size from DPI, with preset devices and formats.
How PPI differs from DPI
PPI counts the pixels in one inch of a screen, DPI counts the dots in one inch of a print. A screen gets its density from its resolution and diagonal: √(width² + height²) divided by the diagonal in inches. A 27-inch 4K monitor works out to 163 PPI, and a 15.6-inch Full HD laptop screen to 141 PPI.
In printing, you set the number yourself. At 300 DPI, a 4000 × 3000 pixel photo takes up 33.9 × 25.4 cm, and at 150 DPI twice that per side. So DPI answers the question of what print size you get without losing quality.
How much you need for printing
Photos and magazines call for 300 DPI, posters do fine at 150, and banners are printed at even lower values, since people view them from a distance. The reverse calculation shows the resolution you need: an A4 sheet at 300 DPI is 2480 × 3508 pixels.
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