300 DPI is a print density, not an image size
At 300 DPI, the calculation assumes 300 pixels for each inch (25.4 mm) of printed length. To print A4 at 300 DPI, the 210 Ă— 297 mm page size is converted to inches before calculating the required pixels.
The tool rounds the final result to whole pixels. A file can say “300 DPI” in its metadata and still be too small in actual pixel dimensions for the intended print size.
A4 works out to about 2480 Ă— 3508 px
A4 portrait at 210 Ă— 297 mm requires about 2480 Ă— 3508 px at 300 DPI. In the other direction, a 2480 Ă— 3508 px image is approximately A4-sized when used at 300 DPI.
To start from an image you already have, use Pixels to print size.
Check the printer or submission requirement before choosing 300 DPI
Around 300 DPI is common for many photo and general print workflows, but required resolution depends on the print process and destination. Do not let this calculator override a printer’s own specification.
Use the 350 DPI calculator or 600 DPI calculator when those are the required values.
Upscaling adds pixels, but it does not recreate missing detail
If an image is too small, resizing can increase its pixel dimensions. That does not automatically restore fine detail that was absent in the source.
A small shortfall may be acceptable in some workflows, but a large shortfall is a reason to look for a better source image or reduce the print size.