How pixel density is calculated
The diagonal in pixels is the square root of width squared plus height squared. Dividing that by the diagonal in inches gives pixels per inch.
Dot pitch is the distance between pixel centres, which is 25.4 millimetres divided by the pixel density. A higher PPI means a finer pitch and, at a normal viewing distance, less visible structure.
Worked example
A 1920 by 1080 panel with a 24 inch diagonal has 2202.91 diagonal pixels, which is 91.79 pixels per inch. The dot pitch is 0.2767 millimetres and the panel measures 531.3 by 298.9 millimetres, or 2.07 megapixels.
These figures use the visible panel diagonal, not the size of the monitor including its bezel.
What PPI does and does not tell you
PPI describes the panel, not the sharpness you perceive. Viewing distance matters as much: a 4K phone at 400 PPI and a 4K television at 80 PPI both look sharp from their intended distance.
Scaling also matters. A high PPI screen run at 150 per cent scaling shows the same amount of content as a lower PPI screen at 100 per cent, only smoother.
Limits of this calculator
It assumes square pixels and the full panel diagonal. It does not account for a curved panel's effective viewing geometry, subpixel layout or the difference between a quoted and a measured diagonal.
For the shape of the panel rather than its density, use the aspect ratio calculator.
For the shape of the panel rather than its density, use the aspect ratio calculator.
Inputs for the PPI calculator
Horizontal pixels
The native horizontal resolution of the screen, not a scaled setting.
Vertical pixels
The native vertical resolution of the same screen.
Diagonal size
The quoted diagonal in inches, which measures the visible panel rather than the bezel.
CALCULATOR QUESTIONS
PPI calculator questions
Is PPI the same as DPI?
Not quite. PPI describes pixels on a screen; DPI describes dots of ink a printer lays down. The terms are often used loosely.
Does higher PPI always look better?
Up to a point. Beyond what your eye resolves at your viewing distance, more density buys little.
Why does my monitor's spec sheet differ?
Manufacturers often round the diagonal to a whole inch, which shifts the calculated density slightly.
What is dot pitch?
The distance between adjacent pixel centres, the reciprocal of pixel density once converted to millimetres.