How do you calculate screen PPI?

Screen PPI (pixels per inch) equals the diagonal pixel count divided by the diagonal screen size in inches. The diagonal pixel count is the square root of (horizontal pixels squared plus vertical pixels squared). For example, a 1920x1080 screen on a 24-inch diagonal: diagonal pixels = sqrt(1920^2 + 1080^2) = 2202.9 pixels. PPI = 2202.9 divided by 24 = 91.79 PPI.

What is the difference between DPI and PPI?

PPI (pixels per inch) refers to screen pixel density: how many pixels fit into one inch of a display. DPI (dots per inch) technically refers to the dot density of a physical printer: how many ink dots are placed per inch when printing. In practice the terms are often used interchangeably, particularly when specifying the required resolution of a digital image intended for printing.

How many pixels do I need for a 300 DPI print?

Multiply the print size in inches by the DPI to get the required pixel count. For a 4-inch by 6-inch print at 300 DPI: width = 4 times 300 = 1200 pixels; height = 6 times 300 = 1800 pixels. A total of 1200 times 1800 = 2,160,000 pixels, which is about 2.16 megapixels. Most cameras easily exceed this for small to medium prints.

DPI and PPI Calculator

Pixel density tells you how sharp a screen looks or how high a resolution an image needs to be for a given print size. The two standard measures are PPI (pixels per inch) for screens and DPI (dots per inch) for print output, though the terms are often used interchangeably in everyday conversation. For screens, a higher PPI means individual pixels are harder to see from normal viewing distance, producing a sharper image. A standard desktop monitor is typically 90 to 110 PPI, a high-definition laptop display sits around 150 to 200 PPI, and a phone screen may exceed 400 PPI. For print, 300 DPI is the standard for photographic quality output, while 150 DPI is acceptable for documents viewed from reading distance and 72 DPI is sufficient for screen-only images. This calculator works in two modes. Screen PPI mode takes the horizontal and vertical pixel count of the screen and the diagonal screen size in inches, and returns the PPI, dot pitch in millimetres, and the diagonal pixel count. Print DPI mode takes the desired output DPI and the print width and height in inches, and returns the pixel dimensions required. You switch between modes using the selector below. The default is screen PPI mode with a 1920 by 1080 resolution on a 24-inch diagonal, which represents a typical desktop monitor.

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Calculation mode

Horizontal resolution

Vertical resolution

Diagonal screen size

inches

91.79 PPI

screen pixel density

Dot pitch0.277 mm

Diagonal pixels2,203 px

Total pixels2,073,600 px

How it works

For screen PPI: diagonal pixel count = sqrt(W² + H²) where W and H are horizontal and vertical pixel counts. PPI = diagonal pixel count divided by diagonal screen size in inches. Dot pitch in mm = 25.4 divided by PPI. Total pixels = W times H. For print DPI: required pixel width = print width (in) times DPI; required pixel height = print height (in) times DPI. Total pixels = width times height.

Worked example

Screen mode: a monitor has a resolution of 1920 times 1080 pixels on a 24-inch diagonal. Diagonal pixels = sqrt(1920² + 1080²) = sqrt(3,686,400 + 1,166,400) = sqrt(4,852,800) = 2,202.9 pixels. PPI = 2202.9 divided by 24 = 91.79 PPI. Dot pitch = 25.4 / 91.79 = 0.277 mm. Total pixels = 1920 times 1080 = 2,073,600. For the print mode: 300 DPI on a 4 by 6 inch print requires 4 times 300 = 1200 px wide and 6 times 300 = 1,800 px tall.

Related calculators

Data Transfer Rate Converter: convert streaming or download speeds to relate to video resolution requirements.
Scientific Notation Converter: convert megapixel counts and large pixel dimensions between notation formats.
Percent Error Calculator: compare measured PPI to a manufacturer's specification.
Measurement Uncertainty Calculator: quantify variation in repeated screen size measurements.

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