Start with the task
Pixel dimensions describe how many samples the image contains. Print density describes how closely those samples are placed. Changing only a PPI number does not create new detail. Resampling changes the pixel dimensions and can introduce interpolation artifacts.
Work through the checks
For a proposed print, divide pixels by pixels per inch to obtain inches; multiply inches by 2.54 for centimetres. A 3000-pixel edge at 300 PPI measures 10 inches, or 25.4 cm. This is arithmetic, not a guarantee that every image will look sharp. Focus, noise, viewing distance and printing process still matter.
Before delivering
Keep the original, select the intended aspect ratio and resize a copy once for the destination. If an image must fill a different ratio, decide whether to crop or add borders rather than stretching it. Compare the exported file at final size. Browser canvas tools are convenient for delivery copies, not for preserving a complete RAW or colour-managed print workflow.
