DPI and PPI are often used interchangeably, but they describe different parts of an imaging workflow.
Pixels per inch (PPI) describes how many image pixels fit into one inch. It is commonly used for screens, image files, and print layout.
Dots per inch (DPI) describes how densely a printer places ink dots. The effective DPI depends on the printer, its settings, and the printing process.
A numeric 1:1 relationship is sometimes used as a practical convention, but it is not a conversion formula. To prepare an image for print, use its pixel dimensions and the desired physical print size to determine the PPI you need.