How is PPI calculated?

PPI = √(width² + height²) ÷ diagonal-in-inches. For a 2560×1440 screen on a 27-inch panel that is √(2560² + 1440²) ÷ 27 ≈ 108.8 PPI.

What is the difference between DPI and PPI?

PPI (pixels per inch) describes screen pixel density, while DPI (dots per inch) describes printer dot density. For displays the terms are used interchangeably, and this tool computes pixels per inch.

What is a good PPI for a monitor?

It depends on viewing distance. Desktop monitors around 90-110 PPI look fine at arm's length, while 'Retina' phones and high-DPI laptops exceed 200-300 PPI because they are viewed up close. Higher PPI means sharper text and images.

Dot pitch is the distance between the centres of neighbouring pixels, equal to 25.4 ÷ PPI millimetres. A smaller dot pitch means a sharper image.

Does a bigger screen always have lower PPI?

For the same resolution, yes — a 27-inch 1440p screen has lower PPI than a 24-inch 1440p screen because the same pixels are spread over more inches. Increasing the resolution alongside the size keeps PPI high.

Why use the diagonal rather than width and height in inches?

Screens are specified by their diagonal size (e.g. a 27-inch monitor), so the calculator combines that with the pixel diagonal — √(width² + height²) — to find density. It then derives the physical width and height for you.

What is the DPI / PPI Calculator?

Free DPI / PPI calculator — work out the pixel density of any screen from its resolution and diagonal size in inches, plus dot pitch and active display area. Runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

DPI / PPI Calculator

Name: DPI / PPI Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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DPI / PPI calculator

Work out the pixel density of any monitor, phone, or tablet from its resolution and diagonal size. It is useful for comparing screen sharpness across products, checking whether a display qualifies as high-DPI (“Retina”), or deciding what monitor resolution you actually need at a given desk distance.

How it works

Screens are marketed by their diagonal size, so the calculation combines the pixel diagonal with that physical measurement. The Pythagorean theorem gives the pixel diagonal, which is then divided by the inch diagonal to yield PPI:

PPI = √(width² + height²) ÷ diagonal in inches

From the PPI, the tool also derives:

Dot pitch = 25.4 ÷ PPI — the distance in millimetres between neighbouring pixel centres. Smaller dot pitch means sharper output.
Active display area — the physical width and height in both inches and centimetres, derived from pixel dimensions divided by PPI.

Worked example

For a 2560 × 1440 display on a 27-inch panel:

diagonal pixels = √(2560² + 1440²) = √(8,627,200) ≈ 2937.8
PPI = 2937.8 ÷ 27 ≈ 108.8
dot pitch = 25.4 ÷ 108.8 ≈ 0.234 mm

Pixel density reference — common screens

Screen	Resolution	Diagonal	Approx. PPI
27” QHD desktop	2560 × 1440	27”	~109
24” 1080p desktop	1920 × 1080	24”	~92
27” 4K desktop	3840 × 2160	27”	~163
15.6” 4K laptop	3840 × 2160	15.6”	~282
6.1” 1080p phone	1080 × 2400	6.1”	~432

What PPI means in practice

The right pixel density depends on viewing distance, not an absolute number. At a typical desk distance of about 60–70 cm, most people cannot distinguish individual pixels above roughly 110–120 PPI — which is why 4K at 27 inches (~163 PPI) appears noticeably crisper than 1080p at 27 inches (~82 PPI), but both look smoother than a phone held at arm’s length needs to.

Apple’s original “Retina” threshold was approximately 300 PPI at phone-in-hand distance (~25 cm). For laptop screens viewed at ~50 cm, Apple uses ~200–220 PPI. For desktop monitors at ~60–70 cm, they target ~110 PPI. These are useful calibration points when choosing a display for a specific use case.

DPI vs PPI — what’s the difference?

PPI (pixels per inch) is the correct term for screen pixel density. DPI (dots per inch) technically describes printer resolution, where each ink dot is smaller than a pixel. For displays the two terms are used interchangeably in everyday language, but this calculator computes pixels per inch. Everything runs in your browser — nothing is uploaded.