How to convert pixels to centimeters
A photo has no size in centimeters until you set a print resolution. The same 6000 × 4000 px file prints at 50.8 × 33.9 cm at 300 DPI and twice that, 101.6 × 67.7 cm, at 150 DPI. Formula: centimeters = pixels / DPI × 2.54. The other way round: pixels = centimeters / 2.54 × DPI.
DPI is the number of dots per inch (2.54 cm). Use 300 DPI for prints and albums viewed up close, 150 DPI is enough for posters and canvases viewed from a meter away, and on screen only the pixel count matters, because a monitor shows one pixel per pixel anyway.
What photo size you need for printing
A 10 × 15 cm print at 300 DPI needs 1181 × 1772 px, about 2 Mpx. A4 (21 × 29.7 cm) is 2480 × 3508 px, about 8.7 Mpx. A 50 × 70 cm poster at 150 DPI is 2953 × 4134 px, about 12 Mpx. A 12 Mpx camera covers prints up to 20 × 30 cm at 300 DPI, and up to A3 at 150 DPI.
Changing the DPI in an image editor without resampling the pixels changes neither the file nor its quality, only the stored print size. The lab looks at the pixel count and format anyway. With too few pixels the image gets stretched and loses sharpness; with too many it gets scaled down without loss.
Camera and phone photos for print
A 24 Mpx camera (6000 × 4000 px) prints 50.8 × 33.9 cm at 300 DPI, a 12 Mpx phone (4000 × 3000 px) prints 33.9 × 25.4 cm. The problem is not the camera but the sending: messengers and social media shrink photos to 1 to 2 Mpx, which makes a good print of 13 × 18 cm at most.
So send photos for print to your client in full resolution, through a gallery or a file, not through a chat. In Picvelo clients download the original files straight from the gallery, and you see which photos they picked for print.