Print size and aspect ratio
Most cameras shoot at 3:2. Prints in 10 × 15 cm (4 × 6 in), 20 × 30 cm (8 × 12 in) and 40 × 60 cm have the same ratio, so the photo fills them completely with no cropping. Phones and compact cameras usually shoot 4:3, which fits 30 × 40 cm.
Sizes at 7:5 (3.5 × 5 in, 5 × 7 in, 15 × 21 cm, 50 × 70 cm), 5:4 (40 × 50 cm) and the A series (A4, A3) have a different shape than a 3:2 frame. The photo is then cropped at the edges or white margins stay on the paper. Cropping does not change resolution but removes part of the frame, so leave some room around the edges when you compose for print.
What size is a 4 × 6 print (10 × 15 cm)?
A 4 × 6 in print, sold in Europe as 10 × 15 cm, has a 3:2 ratio, the same as the frame from most cameras. At 300 DPI it needs 1800 × 1200 px (landscape), and half of that in each direction at 150 DPI. The exact size is 10.2 × 15.2 cm because the format comes from inches.
How many pixels you need to print
300 DPI is the value for prints viewed up close: photo prints, albums and photo books. Large prints viewed from a few meters look good at 150 DPI, and very large ones need even less. Check your own combination with the DPI calculator and Pixels to inches and cm tools.
A4 print
A4 is 21 × 29.7 cm (8.3 × 11.7 in). At 300 DPI it needs 2480 × 3508 px, about 8.7 MP. The A4 ratio is narrower than 3:2, so a camera photo is cropped at the sides or white margins appear.