Pixels Per Inch Calculator
How sharp is that screen, really?
Pixels Per Inch Calculator
PPI = √(width² + height²) / diagonal inches
Calculated instantly
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What This Calculator Does
The Pixels Per Inch Calculator converts a resolution and a diagonal measurement into pixel density, dot pitch, total megapixels, and the aspect ratio. It answers the question a specification sheet hides: how close together are the pixels on this screen?
How It Works
The Pythagorean theorem gives the diagonal in pixels from the width and height. Dividing that by the diagonal in inches gives pixels per inch. Dot pitch is the physical distance between pixel centres, so it is 25.4 mm divided by PPI.
Example
A 3840 × 2160 panel at 27 inches has 4406.9 diagonal pixels, which is about 163.2 PPI. Dot pitch is 25.4 / 163.2 = 0.156 mm, and the panel has 8.29 megapixels at a 16:9 ratio.
Formula
PPI = √(width² + height²) / diagonalDot pitch (mm) = 25.4 / PPIMegapixels = width × height / 1,000,000Reading the Results
Pixels per inch
Physical pixel density. Higher values give smoother text and less visible pixel structure at normal viewing distance.
Dot pitch
The distance between pixel centres in millimetres. It is the reciprocal measure of PPI, so 0.16 mm corresponds to roughly 160 PPI.
Sources and References
W3C — CSS Values and Units Level 4
Reference pixel and device pixel ratio
CSS defines a reference pixel for layout, which is why physical PPI and CSS pixel density are not the same thing.
ISO 9241-307 — Ergonomics of human-system interaction
Display measurement and pixel defect classes
Part 307 covers flat-panel display measurements, including how physical dimensions are specified.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Is PPI the same as DPI?
Not quite. PPI counts pixels in a display or image; DPI counts dots of ink a printer lays down. Marketing often mixes them, but for screens the correct term is PPI.
Does higher PPI always look better?
Up to a point. Beyond roughly 300 PPI at normal viewing distance, most people cannot resolve individual pixels. Contrast, colour accuracy, and viewing distance matter more than a density number alone.
How do I calculate PPI from resolution and diagonal?
Square the horizontal and vertical pixel counts, add them, take the square root to get the diagonal in pixels, then divide by the diagonal in inches. A 1920 × 1080 panel at 24 inches is about 91.8 PPI.
Why does my operating system report a different DPI?
Operating systems use a logical or scaled DPI for layout, especially with display scaling. That number is a UI setting, not the physical density of the panel.