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).

Pixels
PPI

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

20 pixels96 ppi = 0.208 in
Pixel grid visual:
S

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:

Examples of Typical PPI Values for Displays

PPI Description Size (Inches) Pixels (W × H)
50Old CRT monitors, projectors, LED walls16 × 12800 × 600
72Former web standard, classic screen resolution14.2 × 10.61024 × 768
96Windows standard for desktop monitors20 × 11.31920 × 1080
120Full HD laptop displays, low-cost tablets16 × 91920 × 1080
160Compact Full HD displays, tablets13.3 × 7.51920 × 1080
220MacBook Retina Display11.6 × 7.32560 × 1600
264iPad Retina Display9.7 × 7.32048 × 1536
300High-end smartphones (e.g., Android)5.1 × 2.91920 × 1080
326iPhone Retina Display4.1 × 2.31334 × 750
401High-end Android smartphones4.8 × 2.71920 × 1080
500Professional scanners, high-resolution displays10 × 75000 × 3500
600DSLR sensor, professional photo scanners10 × 6.76000 × 4000

Milestones in PPI Development of Displays

Year Development / Event
1984Apple Macintosh (9 inches, ~72 PPI): First clear reference for screen pixel density.
1990sIPS panels introduced: Improved color accuracy and wider viewing angles.
2010Apple iPhone 4 with Retina display (326 PPI): Term for high PPI in smartphones becomes established.
2011LG develops AH-IPS: Higher PPI and improved display quality.
2012Apple introduces Retina displays on iPad and MacBook (up to ~300 PPI).
2014Android flagships exceed 500 PPI for the first time.
2015Everdisplay shows 6" 4K AMOLED with 734 PPI – designed for VR.
2016Google Pixel (441 PPI): Focus on camera and high-resolution displays.
20171440p (QHD) becomes standard in flagships; many devices with >500 PPI.
2018First 4K smartphones (e.g., Sony Xperia XZ2 Premium) exceed 800 PPI.
2019Focus shifts to refresh rate and HDR; PPI stagnates.
2020Apple introduces ProMotion (120 Hz); focus shifts to dynamic frame rates.
2021Micro-LED development progresses: Targeting extreme PPI and high efficiency.
2022Foldable devices become widespread; PPI remains stable at a high level.
2023LTPO technology becomes established for adaptive and power-saving displays.
2024Micro-LED prototypes show over 1000 PPI – designed for VR/AR.
2025Micro-LED mass production begins; extremely high PPI in smartphones & wearables expected.