Use pixels ÷ PPI = inches
The physical print width is pixel width divided by target PPI; the same formula applies to height. Convert inches to centimeters by multiplying by 2.54.
Check aspect ratio before quality
A file can have enough pixels yet still need cropping because its aspect ratio differs from the chosen paper size. Decide the crop before evaluating final effective PPI.
Compare several practical sizes
Instead of deciding from one number, compare 4×6, 5×7, 8×10, 11×14, and larger sizes. The useful output size changes with both source pixels and intended viewing distance.
Avoid solving everything with upscaling
Upscaling can create more pixel samples but cannot recover missing focus or texture. Start with the highest-detail original and resize only when a destination truly requires it.