How to use it
- Type the Horizontal resolution (pixels) and the Vertical resolution (pixels) from the screen's settings or box.
- Type the Diagonal and choose the Unit of the diagonal: inches or centimetres.
- Read the Pixel density (PPI), the Dot pitch (mm) and the Width and Height of the screen.
- Compare with the table Common screens from the Wikipedia article.
How pixel density is worked out
Pixel density, or PPI, is the number of pixels along the diagonal divided by the diagonal in inches. Pythagoras gives the diagonal in pixels: the square root of the width squared plus the height squared. A 1920 by 1080 screen has a diagonal of 2202.9 pixels, so a 24-inch one has 2202.9 ÷ 24 = 91.8 PPI. The Wikipedia article on pixel density gives this two-step method.
The dot pitch is the distance from the centre of one pixel to the next: 25.4 mm, which is one inch, divided by the PPI. The article notes that the unaided eye generally cannot separate detail beyond about 300 PPI, although that depends on the viewing distance.
diagonal (px) = √(width² + height²)
PPI = diagonal (px) ÷ diagonal (in)
dot pitch (mm) = 25.4 ÷ PPIWidth and height of the screen
The diagonal runs from corner to corner, so the width and height follow from the shape of the pixel grid: width = diagonal × width in pixels ÷ diagonal in pixels, and the height in the same way. A 16:9 screen of 24 inches is 20.9 inches wide and 11.8 inches high. The Wikipedia article on display size describes the diagonal as the usual way of stating a screen's size.
What the table shows
The table lists the screens of the Wikipedia article on pixel density: a 15.6-inch laptop at 5120 by 2880, 27-inch and 32-inch screens at 3840 by 2160, and others. Each row uses the same formula as the calculator, so its figures match the article to the rounding the article uses. The article also notes that a monitor's advertised size can differ from its true viewing area: a Dell monitor advertised as 32 inches has 31.5 inches of viewing area, which gives 139.87 PPI rather than 137.68 PPI.
Example: a 24-inch 1080p monitor
A monitor of 1920 by 1080 pixels with a 24-inch diagonal. The diagonal in pixels is √(1920² + 1080²) = 2,202.9 px, so the PPI is 2,202.9 ÷ 24 = 91.8.
Its dot pitch is 25.4 ÷ 91.8 = 0.277 mm. Its width is 20.9 inches, which is 53.1 cm, and its height is 11.8 inches, which is 29.9 cm.
| Measure | Value |
|---|---|
| Pixel density (PPI) | 91.8 |
| Dot pitch | 0.277 mm |
| Diagonal in pixels | 2,202.9 px |
| Width × height | 53.1 × 29.9 cm |
Frequently asked questions
What is a good PPI?
The article on pixel density says the unaided eye generally cannot separate detail beyond about 300 PPI, depending on the distance. Its examples run from 102 PPI for a 21.5-inch 1080p monitor to 577 PPI for the quad HD screen of a Samsung Galaxy S7.
Why does the monitor's size differ from the box?
Makers quote a rounded 'class' size. The article gives a Dell monitor advertised as 32 inches whose viewing area is 31.5 inches. Measure the diagonal of the visible area to get the true PPI.
What does dot pitch mean here?
It is the distance between the centres of two neighbouring pixels, in millimetres. It is 25.4 mm divided by the PPI, since one inch is 25.4 mm. A smaller dot pitch means finer detail on the same size of screen.
Why is the width smaller than the diagonal?
The diagonal runs corner to corner, and the width and height are the two sides of the same shape. On a 16:9 screen the width is about 87 % of the diagonal, and the height about 49 %.
Can this page read my screen's size?
No. The page does not read your screen. You type the resolution and the diagonal from the box or the settings, and only those numbers are used.
Is anything I type sent to a server?
No. The calculation runs in your browser, and nothing you type is stored or sent to us.
Sources
Figures and text last checked on Tuesday, November 10, 2026.