DPI stands for dots per inch and is a unit which is used to represent the resolution of input devices, such as scanners, and of output devices, such as monitors and printers. It is sometimes used in the same sense as PPI (pixels per inch), a unit of image resolution.

When converting an analog image into a digital image (bit map data) using a scanner, the user must decide on the digitization definition. This is called input resolution and is expressed using a unit called DPI. This is represented as the number of pixels per inch but while input resolution is expressed in DPI, image resolution is usually expressed in PPI. However, the image resolution of a bit map data depends on the input resolution, so in this case, DPI and PPI mean the same thing. When setting input resolution, the precision is determined through a process known as sampling to convert analog data into digital data. Since the total number of pixels in the digital image will be defined by the input resolution and magnification at scanning, the image size required for use (total number of pixels) must be considered before scanning the image.

The unit DPI is also used to denote the definition of the output device. When we refer to a “2400dpi printer”, this would mean that the printer is able to print 2,400 dots in an inch. A printer with a higher DPI will, in principle, be able to produce a higher definition output. That said, different types of printers (inkjet, laser, etc.) produce results that appear differently depending on the type of data and what they are used for, so it is not possible to say that a higher resolution printer always produces better looking results. Monitors (display screens) also display images using dots arranged in a grid pattern. There is a set physical screen resolution (number of dots) for each monitor size (such as 15-inch or 17-inch) but the display resolution can be adjusted. The screen resolution of the monitor varies from computer to computer, but this needs to be considered carefully when creating a web page.