How DPI (Dots per Inch) Works in Printing

DPI (dots per inch) is probably the most familiar and most misused measure of resolution. For desktop publishing, it is probably the least important measure as well. It is not the resolution of scanned images. It is not the resolution of images on screen. It is the measure of how many dots of ink or toner a printer can place within an inch (or centimeter).

Most printers print the same number of dots horizontally and vertically, though some may have differing numbers.

Basically, 600 dpi printers print 600 tiny little dots across one inch and 600 dots vertically for one inch.

How SPI, PPI, DPI, and LPI Relate

The lower the DPI of a printer, the less fine detail it can print and the fewer shades of gray it can simulate. Because monitor resolution is so much lower than printers, low-resolution images that look fine on-screen often print poorly.

DPI and Scanning and Printing

DPI is only relevant in that the combination of your printer's DPI and the required LPI determine how much resolution your image must have. If you will be printing to a 300 DPI printer, resolution in excess of 300 SPI is rarely necessary.

In general, the higher the resolution of your printer or imagesetter, the greater detail you can print and the better appearance of your output.

DPI and the Web

On the Web, DPI is irrelevant because images display on-screen in pixels (PPI).

In practice, SPI and PPI are often used interchangeably.

DPI is frequently used in place of one or both terms. However, even if you call it DPI, remember that each dot or "unit of measure" behaves differently depending on whether it is a scanner (or scanned image), a monitor (or on-screen image), or a printer (or printed image).