Print size depends on a relationship between physical dimensions and pixel density. A file can contain the right number of pixels while a print application's fit setting still changes its physical output.
Print Photo Size Tool converts the entered millimetres and DPI into a rounded pixel canvas. It creates JPEG output with resolution information for that DPI.
Work through a familiar print size
At 300 DPI, a 100 by 150 millimetre photo becomes approximately 1181 by 1772 pixels. The calculation divides millimetres by 25.4 and multiplies by the selected dots per inch.
Cover fills the canvas by cropping. Contain retains the whole source and fills remaining space with the chosen background. Decide whether losing edge content or adding space is the acceptable tradeoff.
A larger DPI value needs more output pixels for the same physical dimensions. It does not add genuine detail if the source is too small or blurred.
Printing software has the last scaling choice
Use the intended physical size in the print application's layout and avoid an automatic fit to page option that changes it. Embedded JPEG resolution is useful information, not control over every printer dialog.
The tool limits generated pixel area and dimensions. A very large millimetre size at high DPI can exceed that budget, even if each entered number falls within its own field range.
Check faces, text and cropped edges before printing several copies. For colour critical work, retain the original profile managed file because this tool's fresh encoding does not copy original colour profiles.
Save the chosen millimetres and DPI with the download. Those values explain the intended print layout more clearly than a filename that only says large photo.