Pixels, PPI and the size of a print
Pixels are a count. Centimeters and inches are lengths. PPI is the link between them: how many image pixels are placed in one inch of the print.
The relationship you need
For a whole, uncropped image:
print width in inches = pixel width ÷ PPI
print width in cm = pixel width ÷ PPI × 2.54
A 3,000 pixel-wide image printed at 300 PPI is 10 inches, or 25.4 cm, wide. Print that same file 15 cm wide and its effective resolution becomes 508 PPI. The pixel count did not change; the pixels are simply placed more closely together.
What changing a DPI label does
An image editor can change a resolution metadata value without adding pixels. That changes the suggested print size, not the amount of captured detail. Resampling changes pixel dimensions, but adding interpolated pixels does not recreate missing detail from the original scene.
People often say DPI when discussing an image. Strictly, image density is PPI (pixels per inch); a printer’s marking resolution is DPI (dots per inch). One image pixel may be represented using several printer dots. The checker calculates image PPI.
Account for the crop
A 3:2 photo placed in a square frame must either lose its sides or leave white space. Fill mode crops the source before calculating effective PPI. Fit mode preserves the source and reports the smaller area the image actually occupies.
Start with 3,000 × 2,000 pixels and fill a 10 × 10 cm square. A centered crop uses 2,000 × 2,000 pixels. Effective resolution is 508 PPI, and one third of the source area is removed. Dividing the original 3,000 pixels by the square’s width would overstate the result.
How many pixels are enough?
Choose a target PPI appropriate for your output and viewing distance. 300 PPI is a useful starting input for many small photographic prints, but it is not a guarantee of sharpness or a requirement for every poster. Focus, compression, paper, printing and viewing distance also matter.
The checker’s “minimum pixels” result rounds upward so the requested area reaches at least the selected PPI. For 15 × 10 cm at 300 PPI, that is 1,772 × 1,182 pixels. A conventional table that rounds to the nearest integer may differ by one pixel.
Use the result in a layout
Once the size and crop suit your image, use Use this size in a print sheet. The next tool carries over the physical dimensions. You then open your image locally, choose paper and copies, and save or print the sheet.
References
Adobe Photoshop: image size and resolution
W3C: CSS units and physical lengths