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Screen PPI Calculator (Pixel Density)
Enter a screen's resolution and its diagonal size to calculate pixels per inch. The result also gives the dot pitch, the physical width and height, the megapixel count and a rule-of-thumb viewing distance.
The PPI formula
Pixels per inch (PPI) is how many pixels fit along one inch of the screen. Because the size of a screen is quoted along its diagonal, the calculation uses the Pythagorean theorem on the pixel grid first:
diagonal in pixels = √(width² + height²)
PPI = diagonal in pixels ÷ diagonal in inches
As a check, a 300 × 400 pixel panel has a 500-pixel diagonal, so on a 5-inch screen it gives exactly 100 PPI. A 27-inch 2560 × 1440 monitor has a diagonal of about 2937 pixels, giving 108.8 PPI, while a 27-inch 3840 × 2160 monitor reaches about 163.2 PPI.
Dot pitch is the reverse view: the size of a single pixel, which equals 25.4 mm ÷ PPI.
What a PPI number tells you
Higher PPI means smaller pixels and finer detail, as long as you view the screen from the same distance. Text edges, icons and photographs look smoother, and individual pixels become harder to see. But PPI alone does not decide sharpness. A phone is held closer than a monitor, so a phone needs far more PPI to look equally crisp.
The viewing distance line uses the idea that a person with typical eyesight can resolve detail of about one arcminute. At 3438 ÷ PPI inches, one pixel subtends that angle, and beyond it the pixel grid is generally not discernible. It is only a guide: eyesight varies, and thin lines or text can reveal pixels at greater distances.
PPI, DPI and scaling
- PPI versus DPI. PPI describes screens and digital images, and image editors also use it to label the intended print size. DPI (dots per inch) describes the dots a printer lays down, so the two are not interchangeable even though people mix them.
- Diagonal size is nominal. Manufacturers quote the visible diagonal, and rounded values such as 6.1 or 27 can hide a small difference from the real measurement, so published PPI figures may differ a little from the value computed here.
- Operating system scaling. A high-PPI display is often run at 150% or 200% scaling so interface elements keep a comfortable size. Scaling changes what is drawn, not the panel's physical density.
Limits
The calculation assumes square pixels and a flat panel. Curved screens, non-standard pixel layouts and projectors do not follow the formula in the same way, and the physical size of a projected image depends on distance. For the shape of the screen alone, the aspect ratio calculator is the better tool.
When comparing two displays, compare PPI at the distance you will actually sit. A 14-inch laptop at 1920 × 1080 has a higher density than a 27-inch monitor at the same resolution, yet the monitor is viewed from further away, so the two can appear equally sharp in daily use.
Worked example
A 27-inch monitor with a 3840 × 2160 (4K) panel. How dense are its pixels compared with a 1440p monitor of the same size?
Inputs
| Horizontal resolution | 3840 px |
|---|---|
| Vertical resolution | 2160 px |
| Screen diagonal | 27 |
| Diagonal is in | Inches |
Result
Diagonal pixels divided by diagonal inches.
| Pixels per centimetre | 64.2 |
|---|---|
| Dot pitch (pixel size) | 0.1557 mm |
| Diagonal in pixels | 4,405.8 |
| Physical width | 23.53 in (59.8 cm) |
| Physical height | 13.24 in (33.6 cm) |
| Total pixels | 8,294,400 (8.29 megapixels) |
| Aspect ratio | 16:9 |
| Distance where one pixel spans one arcminute | 21.1 in (54 cm) |
- The viewing distance is a rule of thumb based on typical visual acuity of one arcminute. Individual eyesight and content type change what looks sharp.
Accuracy note: Results are calculated in your browser from the values you enter. Check anything that matters for legal, academic or financial purposes against the official source.
Frequently asked questions
How do I calculate PPI?
Find the diagonal in pixels with the Pythagorean theorem, then divide by the diagonal in inches. For a 1920 by 1080 screen of 24 inches that is about 2203 pixels divided by 24, or 91.8 PPI.
What PPI is considered sharp?
It depends on viewing distance. Phones held close usually need several hundred PPI, while a desktop monitor viewed from further away can look sharp at about 100 to 160 PPI.
Is PPI the same as DPI?
No. PPI counts pixels on a screen or in an image, while DPI counts printed dots from a printer. They are often confused, but they measure different things on different devices.
Why does my result differ from the manufacturer figure?
Quoted diagonals are rounded and sometimes include a small margin. Rounding in the diagonal size changes the PPI by a fraction, which accounts for most small differences.
Does a higher resolution always mean a higher PPI?
Not by itself. PPI depends on the resolution and the physical size together. The same 1080p panel is far sharper on a 15-inch laptop than on a 55-inch television.