Finds the printed size of an image from its pixel dimensions and resolution. Resolution is pixels per inch, so dividing the pixel count by the dpi gives the printed length. Photographic printing usually aims at 300 to 350 dpi.
This tool finds how large an image comes out on paper, given its pixel dimensions and the resolution it is printed at.
Here is the number of pixels, the resolution in dots per inch and the printed length in millimetres. Resolution is pixels per inch, so dividing the pixel count by the dpi gives inches, and 25.4 converts inches to millimetres.
Photographic printing aims at 300 to 350 dpi. Below that, the grain shows when the print is held in the hand. Work meant to be seen from a distance, such as a poster, is fine at 150 dpi, and anything shown only on screen needs 72 to 96.
A 4000 × 3000 image printed at 350 dpi measures 290.3 mm by 217.7 mm, and holds 12 megapixels.
4000 ÷ 350 = 11.43 inches, and multiplying by 25.4 gives 290.3 mm. A sheet of A4 is 210 × 297 mm, so the image more than fills it.
Resolution is not a property of the image itself. The same pixel dimensions give any dpi you like, depending on the size printed. What the image really carries is its pixel count.
Enlarging a file to add pixels does not restore detail that was never captured.