How are pixels and print size related?
Physical size in inches multiplied by DPI gives the pixel count along one dimension. The calculation can use centimetres or inches for the physical measurement.
What does DPI not determine?
DPI/PPI describes density at a stated physical size. It does not improve image detail, color quality, crop composition or printer capability.
Common print sizes at 300 DPI
A4 (210 × 297 mm, 8.27 × 11.69 in) needs about 2480 × 3508 pixels. A 6 × 4 in photo needs 1800 × 1200 pixels, and a US letter page (8.5 × 11 in) needs 2550 × 3300 pixels. At 150 DPI, often enough for posters viewed from a distance, each figure halves.
Worked example
A 4000 × 3000 pixel photo printed at 300 DPI comes out 13.33 × 10 inches (33.9 × 25.4 cm). To print the same photo 60 cm wide, the density would be 4000 / (60 / 2.54) ≈ 169 DPI, which is usually acceptable for a wall print viewed from arm's length.
DPI or PPI?
PPI (pixels per inch) describes an image or screen; DPI (dots per inch) strictly describes printer dots. Image software and print shops use the two interchangeably when they ask for a 300 DPI file, meaning 300 image pixels per printed inch. Changing the DPI field in an image's metadata alone does not add pixels or detail.