Pixel density

In many applications, such as Adobe Photoshop, the program is designed so that one creates new images by specifying the output device and PPI (pixels per inch).

Consider a 23″ HD monitor (20″ wide), that has a known, native resolution of 1920 pixels (horizontal).

Let us assume an artist created a new image at this monitor resolution of 1920 pixels, possibly intended for the web without regard to printing.

Rewriting the formula above can tell us the pixel density (PPI) of the image on the monitor display: Now, let us imagine the artist wishes to print a larger banner at 48″ horizontally.

Since a printer device is capable of printing at 300 ppi, the resolution of the original image is well below what would be needed to create a decent quality banner, even if it looked good on a monitor for a website.

[3] Often one wishes to know the image quality in pixels per inch (PPI) that would be suitable for a given output device.

[4] Using the DPI or LPI of a printer remains useful to determine PPI until one reaches larger formats, such as 36" or higher, as the factor of visual acuity then becomes more important to consider.

The dot pitch of a computer display determines the absolute limit of possible pixel density.

[8] Some observations indicate that the unaided human generally can't differentiate detail beyond 300 PPI.

The human eye also responds in a different way to a bright, evenly lit interactive display from how it does to prints on paper.

PPI can be calculated from the screen's diagonal size in inches and the resolution in pixels (width and height).

[18] Sony's Xperia XZ Premium has a 4K display with a pixel density of 807 PPI, the highest of any smartphone as of 2017.

[21][22][23] The PNG image file format only allows the meter as the unit for pixel density.

[24] The following table show how pixel density is supported by popular image file formats.

As the table shows, support for density information in image file formats varies enormously and should be used with great care in a controlled context.

The outside of the square shown above is 200 pixels by 200 pixels. To determine a monitor's ppi, set the OS DPI scaling setting at 100% and the browser's zoom at 100%, then measure the width and height, in inches, of the square as displayed on a given monitor. Dividing 200 by the measured width or height gives the monitor's horizontal or vertical ppi, respectively, at the current screen resolution.