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Sensors, Pixels and Image Sizes

Pixels and Print Sizes

Printer resolutions are usually specified by the number of dots per inch (dpi) that they print while images and display screens are specified by pixels per inch ppi.

For comparison purposes, monitors usually use somewhat less than 100 ppi to display text and images, inkjet printers range up to 4800 dpi or so, and commercial typesetting machines range between 1,000 and 2,400 dpi.

Understanding printer resolutions is complicated by their advertised resolutions. The advertised dpi refers to the number of individual dots of ink the printer can print per inch. However, anywhere dots from 4 to 12 colors are needed to print a pixel of a specific color. For this reason, the ppi is always dramatically lower than the dpi. For most purposes, digital images print best at 200 or 300 ppi.

Since image sizes are described in pixels and photographic prints in inches, you sometimes have to convert between these units. To do so, you divide the image's dimension in pixels by the resolution of the device in dots per inch (dpi). For example, to convert the dimensions for a 1500 x 1200 image being printed at 300 ppi you divide as follows:

The result is a 5" x 4" print. However, if the output device prints 600 dpi, the print size falls to 2.5" x 2" as follows:

This graphic shows the relative sizes of a 3000 x 2000 image printed or displayed on devices with different dots per inch. At 72 dpi it's 41.7" x 27.8", at 300 dpi it's 10" by 7", and at 1500 dpi, it's only 2" x 1.3"—a little larger than a stamp.

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