PPI describes the image
Pixels per inch (PPI) describes how many image pixels are assigned to each printed inch. A 3000-pixel-wide image printed 10 inches wide has an effective resolution of 300 PPI.
3000 pixels ÷ 10 inches = 300 PPI
DPI describes the printer
Dots per inch (DPI) describes the physical ink or toner dots a printer can place. A printer may use several ink dots to reproduce one image pixel, so printer DPI and image PPI are not a one-to-one measurement.
Why people still say 300 DPI image
In everyday design and photo-printing conversations, DPI is frequently used when PPI is technically intended. If a printer asks for a 300 DPI image, they generally mean that your file should contain enough pixels to produce 300 pixels per printed inch at the final size.
Practical targets
- 300 PPI: detailed photographs and artwork viewed closely
- 200-240 PPI: very good general photo printing
- 150 PPI: posters and larger prints viewed from farther away
- Below 100 PPI: signage or distant viewing, depending on the image
Use effective PPI
The useful number is the PPI at the final print size, not a metadata value viewed in isolation.