PPI Calculator
A 27-inch 2560 x 1440 monitor has 108.8 PPI and a dot pitch of 0.233 mm. A 27-inch 4K (3840 x 2160) monitor has 163.2 PPI, so pixels blend together beyond about 21 inches.
How to use
- Enter the screen diagonal in inches (or press cm), then the horizontal and vertical resolution in pixels, such as 2560 and 1440. Or press a preset.
- Read the PPI at the top. Below it are the dot pitch, the real width and height of the display, pixels per centimetre, aspect ratio, megapixels and the retina distance.
- Compare screens by entering each one. A higher PPI gives a sharper picture at the same viewing distance.
- Use the Share link button to send your exact values to someone.
How it works
PPI = sqrt(width px squared + height px squared) / diagonal in inches. The square root is the diagonal of the screen measured in pixels, by the Pythagorean theorem.
Dot pitch in mm = 25.4 / PPI. It is the distance between the centres of two neighbouring pixels. Pixels are assumed square.
Screen width = diagonal x width px / diagonal px, and height likewise, so the physical size follows from the pixel aspect ratio. The result is the visible panel area, without the bezel. Rounding in the stated diagonal gives a few millimetres of error.
Retina distance = (1 / PPI) / tan(1 arcminute), which is about 3438 / PPI inches. At this distance one pixel covers 1/60 of a degree, the smallest detail a 20/20 eye can resolve. Farther away, individual pixels cannot be told apart. It is a rule of thumb, not a hard limit: some people see finer detail, and thin lines and text edges can still show aliasing.
PPI of common screens
Retina distance is the viewing distance beyond which one pixel is smaller than the eye can resolve, assuming 20/20 vision.
| Screen | PPI | Dot pitch | Retina distance |
|---|---|---|---|
| 24 in Full HD | 91.8 | 0.277 mm | 37.5 in (95 cm) |
| 27 in QHD | 108.8 | 0.233 mm | 31.6 in (80 cm) |
| 27 in 4K | 163.2 | 0.156 mm | 21.1 in (54 cm) |
| 32 in 4K | 137.7 | 0.184 mm | 25.0 in (63 cm) |
| 13.3 in laptop 2560 x 1600 | 227.0 | 0.112 mm | 15.1 in (38 cm) |
| 6.1 in phone 2532 x 1170 | 457.3 | 0.056 mm | 7.5 in (19 cm) |
| 55 in 4K TV | 80.1 | 0.317 mm | 42.9 in (109 cm) |
Standards
The retina distance rests on the convention that normal (20/20) vision resolves a detail of 1 arcminute. The conversion 1 inch = 25.4 mm is exact by definition.
| Document | Defines | Where |
|---|---|---|
| NIST SP 811 | Exact definition of the inch: 25.4 mm | https://www.nist.gov/pml/special-publication-811 |
FAQ
What is a good PPI?
It depends on viewing distance. Around 110 PPI looks sharp at desk distance for most people, 160 or more looks very sharp, and phones are 400 or more because they are held close. Sharpness depends on PPI and distance together, which is what the retina distance shows.
Is PPI the same as DPI?
No. PPI counts pixels on a screen. DPI counts printed dots per inch. People often use them loosely, but a 300 DPI print and a 300 PPI screen are not the same thing.
Why does the same resolution look sharper on a smaller screen?
The same number of pixels is packed into a smaller area, so PPI is higher. 1920 x 1080 gives 91.8 PPI on 24 inches but 141 PPI on 15.6 inches.
How is retina distance worked out?
It is the distance at which a single pixel spans one arcminute (1/60 of a degree), the resolving limit of normal 20/20 vision. It is about 3438 divided by the PPI, in inches. Apple used the same idea when it named its high-density displays Retina.
Why is my screen size slightly different from the number on the box?
Manufacturers round the diagonal, and some quote the size including a curved edge or a bezel. Measure the visible diagonal with a ruler if you need it exact.
Does it work for non-square pixels?
No. The calculation assumes square pixels, which is true for nearly all current monitors, phones and TVs.