PPI (Pixels Per Inch) and DPI (Dots Per Inch) are often used interchangeably, but they describe entirely different technologies. PPI is an input metric belonging to the digital realm; it describes how many square digital pixels exist in an inch of screen space or an image file.
DPI is an output metric belonging strictly to the physical printing process. It describes how many microscopic droplets of physical ink a printer head deposits into a one-inch line on a piece of paper. A printer must use multiple dots of Cyan, Magenta, Yellow, and Black (CMYK) ink to simulate a single digital pixel.
Calculating physical print size using PPI
When preparing an image for print, the industry standard requirement is 300 PPI. This means for every inch of physical paper, the image file must provide 300 digital pixels of data to ensure sharp, photographic quality.
You can calculate the maximum physical print size of any digital image by dividing its pixel dimensions by the target PPI. If you have an image that is 3000 x 2400 pixels, dividing by 300 yields a physical size of 10 inches by 8 inches. Printing that same image at 20x16 inches would stretch it to 150 PPI, resulting in a blurry print.
Why printers need higher DPI than the image PPI
Because a printer only holds four ink colors (CMYK), it cannot print the millions of colors present in a digital file directly. To print a light blue pixel, the printer deposits tiny dots of cyan and white space in a specific halftone pattern. Your eye blends them into light blue.
To replicate a 300 PPI digital image accurately, a high-quality inkjet printer might run at 1200 DPI or even 2400 DPI. It uses dozens of physical ink dots (the DPI) arranged in a grid to accurately render the color of a single digital pixel (the PPI).