What is PPI (Pixels Per Inch)? Definition and Explanation

PPI stands for "Pixels Per Inch" and is a measure of the pixel density of a digital image or display. It indicates how many individual pixels are present within one inch (2.54 cm) of length.

Pixels are the smallest individual dots of a digital grid that make up an image. Depending on the technology, a pixel can represent different color depths (e.g., from 1 bit to over 48 bit).

DPI vs. PPI vs. DPI: A Common Misconception in Image Resolution

Although technically different, the term DPI (Dots Per Inch) is often incorrectly used synonymously with PPI, especially in the context of digital cameras, scanners, and image editing. Even in professional contexts or documentation, DPI is frequently used when the correct term should be image resolution in PPI. It is important to know the difference: DPI refers to printer dots, while PPI refers to the pixels of a digital image.

PPI Characteristics

• Describes the point density on digital displays and in image files.
• Higher PPI means more detail in the same area (sharper image).
• Relevant to perceived image quality, depending on viewing distance.

PPI is a relevant value for devices that display or capture images using a pixel grid:

• Screens (monitors, laptop displays, smartphone displays, etc.)
• Digital cameras (sensor resolution, often indirectly indicated by megapixels)
• Scanners (image capture resolution)
• Projectors (beamers)

An example image with 5 x 5 = 25 pixels. To calculate the PPI, we need the actual physical length (in inches or cm) that these pixels occupy on a display.

How do you calculate PPI?

To calculate the PPI of a display or digital image, you need the number of pixels along an edge and the physical length of that edge in inches. If the length is given in metric units (e.g., mm or cm), it must first be converted to inches (1 inch = 25.4 mm).

PPI can be calculated using the following formula:

PPI = PixelLength (in) = Pixelin = Pixel × 25.4 mmLength (mm)

The following image shows a letter within a 20 pixel field, displayed on a 24" monitor. Based on 96 PPI, the width is calculated in inches. See also: (PPI Calculator):

Formula, image [S] on a 24" monitor at 96 PPI

Difference Between DPI and PPI

Although the calculation method is similar (units per inch), the main difference lies in the details: PPI (Pixels per Inch) refers to the fixed, square pixels of a digital screen or image sensor. DPI (Dots per Inch) typically refers to the physical ink dots of a printer, whose size and spacing may vary (e.g., with dithering). The following image illustrates this difference: