Use a target, not a magic megapixel number
Start with the finished size and a PPI target agreed with your print provider. Multiply each dimension in inches by the PPI. For centimeters, divide by 2.54 first; for millimeters, divide by 25.4. This gives a planning pixel count for that physical size, not an automatic quality grade.
At a chosen 300 PPI, a portrait 4 × 6 inch print corresponds to 1,200 × 1,800 pixels. A 5 × 7 corresponds to 1,500 × 2,100. An 8 × 10 corresponds to 2,400 × 3,000. For landscape, swap the width and height. These figures describe the visible image area before any additional bleed.
Convert metric paper dimensions carefully
A4 measures 210 × 297 mm. At 300 PPI, the calculation is approximately 2,480.315 × 3,507.874 pixels. Rounding to the nearest whole pixel gives a canvas of 2,480 × 3,508 pixels, which is the convention used by these tools. Whole pixels cannot represent every physical dimension exactly. Follow the dimensions in the provider’s template when one is supplied.
Do not assume a 10 × 15 cm product is exactly the same as 4 × 6 inches. The latter measures 10.16 × 15.24 cm. The difference may be small, but it matters when preparing a precise canvas, an existing frame, or a print with a narrow border.
Count the pixels after the crop
A file’s total megapixels do not tell you whether its shape matches the paper. A 3,000 × 2,000 landscape image matches a 6 × 4 inch print. For a 10 × 8 inch fill, keeping its 2,000-pixel height requires cropping the width to 2,500. The retained image then supplies 250 PPI at the chosen print size.
Increasing that cropped result to 3,000 × 2,400 produces a 300 PPI delivery canvas, but it does not reveal new photographic detail. Check both the retained source pixels and the exported dimensions. If the subject is close to an edge, compare a fit layout before committing to the crop.
Finish with the actual order specifications
Ask for the printer’s target resolution, bleed, safe area, color requirements, and accepted format. Add bleed only when requested, using their amount on each edge. Finally, confirm the print dialog or order preview uses the intended physical size. Correct arithmetic prevents sizing surprises; a sharp source and a suitable printing process are still needed for a good result.