Overview

Pixels per inch (PPI) is a measure of pixel density on digital displays and images. It expresses how many individual pixels fit into one linear inch of a screen or scanned image. PPI is closely tied to a device's resolution, the physical size in inches, and the number of pixels in the horizontal and vertical axes: horizontal, vertical and their diagonal direction measurement.

How it is calculated

To compute PPI you divide the length of the diagonal in pixels by the diagonal size in inches. The diagonal in pixels is the square root of the sum of the squares of the horizontal and vertical pixel counts. Expressed as a formula:

  1. PPI = sqrt(width_px^2 + height_px^2) / diagonal_inches
  2. Example: a 1920×1080 panel with a 24" diagonal gives PPI = sqrt(1920² + 1080²) / 24.

The term PPI grew in use as high-resolution displays and digital imaging became common. It is frequently confused with dots per inch (DPI), which more properly describes printer dot density. In scanning and camera contexts, PPI is analogous to scanner sampling and sensor sampling: see scanner and digital camera references for related concepts.

Practical significance and examples

Higher PPI generally produces crisper detail at close viewing distances because individual pixels are smaller and harder to resolve. Typical desktop monitors often fall in the 80–200 PPI range, while many modern smartphones exceed 300 PPI. Perceived sharpness also depends on viewing distance and human visual acuity: beyond a certain point increasing PPI yields diminishing returns for a given use case.

Distinctions and considerations

  • Pixel density vs total resolution: PPI describes how densely pixels are packed, not the total pixel count. A large screen and a small screen can have the same resolution but very different PPI values.
  • DPI vs PPI: Printers use DPI to describe the density of ink droplets; PPI applies to displays and digital images.
  • Scaling and user interfaces: Operating systems and applications often scale fonts and controls according to a display’s PPI so elements remain legible across devices with different densities.

Why it matters

Knowing PPI helps photographers, designers and consumers make informed choices about devices and image preparation. It guides decisions about export sizes, image sharpening, readability of text, and trade-offs between resolution, file size and device cost. For more technical details and comparisons, consult resources on resolution and the links above.