Pixels Per Inch (PPI): what it is, how it works and what it does
PPI (Pixels Per Inch) is a pixel-density metric that expresses how many image points fit along one inch of a screen's diagonal. It is derived from the panel's resolution and physical size and is a standard component of display specifications.
Definition
PPI (Pixels Per Inch) is a pixel-density measure that quantifies how many image points are distributed along one inch of a display's diagonal. A higher value means more detail can be rendered on the same physical surface.
Calculation
PPI is obtained by dividing the diagonal in pixels (computed via the Pythagorean theorem from horizontal and vertical resolution) by the physical diagonal of the panel in inches:
PPI = √(res_h² + res_v²) / diagonal_in_inches
As a concrete example, a 6.67-inch OLED panel with a resolution of 1080 × 2400 px yields approximately 395 PPI: the pixel diagonal is √(1080² + 2400²) ≈ 2632 px, and 2632 / 6.67 ≈ 395.
Place within display specifications
In device datasheets, PPI does not appear in isolation but is embedded in a block of screen parameters. The available evidence shows it listed alongside:
- Resolution and diagonal size (1080 × 2400 px, 6.67")
- Panel technology (OLED) and colour depth (16.7 million colours)
- Refresh rate (120 Hz) and touch sampling rate (240 Hz)
- Peak luminance (2100 cd/m²)
- Contrast ratio (5 000 000:1) and colour-gamut coverage (100 % DCI-P3)
- Screen-to-body ratio (≈ 87.4 %)
This grouping indicates that PPI is a geometric attribute of the panel, complementary to its photometric and colour properties.
Interpreting PPI in a product
When a specification sheet lists a PPI value, the reader can infer:
- The effective resolution the panel delivers for its physical size.
- The apparent fineness of graphical elements: at 395 PPI, a single pixel measures roughly 64 micrometres, which is imperceptible to the naked eye under normal viewing conditions.
- Coherence with the remaining parameters: a high PPI is typically paired with higher luminance and contrast so that the added detail is actually utilised.
Scope and limitations
PPI describes only the geometric density of sub-pixels; it does not, by itself, inform about:
- Panel quality (uniformity, degradation, artefacts).
- Actual colour fidelity, which depends on calibration and the supported colour space.
- Perceived sharpness under varying ambient light or viewing distances.
Moreover, the numerical value depends directly on the stated resolution and diagonal size; a change in either parameter alters the PPI without necessarily changing the subjective impression of sharpness.
In summary, PPI is a descriptive, reproducible metric that allows comparison of image density across screens of different sizes and resolutions, but it must be interpreted together with the rest of the display's specifications to assess overall performance.