Pixel Density (PPI) Calculator
Enter the resolution and diagonal size of a phone, monitor or TV to get its PPI, dot pitch and the approximate viewing distance at which individual pixels are no longer visible.
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Formula
PPI = sqrt(width px^2 + height px^2) / diagonal (in)
dot pitch (mm) = 25.4 / PPI
retina distance = 1 / tan(1 arcminute) / PPI = about 3438 / PPI inches
megapixels = width px x height px / 1,000,000
The retina distance is a rule of thumb based on 20/20 vision resolving about one arcminute. It is an approximation, not a guarantee that no pixel is visible to anyone. Pixels are assumed square.
Worked example
A 27 inch 2560 x 1440 monitor:
- diagonal in pixels = sqrt(2560^2 + 1440^2) = 2937.3
- PPI = 2937.3 / 27 = 108.8
- dot pitch = 25.4 / 108.8 = 0.233 mm
- retina distance = 3437.7 / 108.8 = 31.6 in (80.3 cm)
- pixels = 2560 x 1440 = 3.69 megapixels
Frequently asked questions
How is PPI calculated?
PPI = sqrt(width px^2 + height px^2) / diagonal in inches. A 2560 x 1440 screen of 27 inches has a diagonal of 2937.3 pixels, so 2937.3 / 27 = 108.8 PPI.
What is dot pitch?
Dot pitch is the distance between neighbouring pixels in millimetres: 25.4 / PPI. At 108.8 PPI it is 0.233 mm. Lower means a sharper image.
What is the retina viewing distance?
It is a rule of thumb, not a hard limit. Normal vision resolves about 1 arcminute, so pixels blend together beyond roughly 3438 / PPI inches. At 108.8 PPI that is 31.6 inches (80 cm). Eyesight and content vary.
What PPI is considered high?
Around 90 to 110 PPI is typical for desktop monitors and about 140 or more looks sharp at desk distance. Phones are held closer, so 400 PPI and above is common: a 1170 x 2532 phone at 6.1 inches is 457 PPI.