Enter the width and height in pixels. See how many centimetres stay sharp for each use, and whether A4 is safe.
Dots per inch — how many dots fit in one inch (2.54 cm). The same pixels print small and sharp at high DPI, or large and coarse at low DPI. The formula is simple: print size (inches) = pixels ÷ DPI.
Things held in the hand — cards, flyers — need 300 DPI. Posters are viewed at arm's length, so 150 is plenty; banners from metres away work at 72; billboards are made at 30. The farther away, the lower it can be — which is why one photo can be fine for a banner but not a business card.
300 DPI is near the limit of what the eye resolves at 30 cm. Beyond that, files grow with no visible gain. Some presses ask for 350, so check before ordering.
The DPI stored in an image file is just metadata; it does not change quality. A 4000 px photo tagged 72 DPI and one tagged 300 DPI are the same photo. Pixel count is what matters.
A recent phone (12 MP, 4032×3024) fills A4 above 300 DPI — yes. A cropped, zoomed shot has fewer pixels and may not.
Upscaling invents no new detail; it only blurs. The only real fix is a larger original.
Read the table backwards. A4 (21×29.7 cm) at 300 DPI needs 2480×3508 px. Common sizes are in the preset buttons.