PPI Calculator

How the PPI formula works

Pixel density is simply the diagonal resolution divided by the diagonal size. Because screens are rectangles, the diagonal resolution is found with the Pythagorean theorem: √(width² + height²). So the full formula is PPI = √(width² + height²) ÷ diagonal-in-inches. A 3840×2160 (4K) monitor with a 27-inch diagonal yields √(3840² + 2160²) ÷ 27 ≈ 163.18 PPI.

Total pixels is just width × height — useful for understanding raw sharpness budget and, incidentally, the graphics workload a display demands. A 4K panel has about 8.3 megapixels, roughly four times a 1080p panel's 2.1 megapixels.

Aspect ratio is the width-to-height proportion reduced to its lowest terms via the greatest common divisor. 1920×1080 reduces to 16:9, and a phone's 1170×2532 reduces to 195:422 (marketed as ~19.5:9). The reduced ratio tells you the shape of the screen, independent of its absolute resolution.

When judging results, keep viewing distance in mind. The classic “Retina” figure of 300 PPI applies to phones held about 30 cm away. A desktop monitor at arm's length is comfortable at 100–160 PPI, and a laptop at 220–270 PPI. In other words, the same PPI value means different things for different device classes — context matters as much as the raw number.

One practical trick: if you know a display's PPI and its resolution, you can recover its exact diagonal — diagonal pixels ÷ PPI gives the size in inches. The numbers are fully interchangeable, which is what makes PPI such a useful single figure: it collapses resolution and size into one density measure that is directly comparable across every device class.

Try a few examples: a 24″ 1080p monitor gives about 92 PPI, while a 27″ 4K monitor gives about 163 PPI; a 6.1″ phone at 1170×2532 gives about 457 PPI. Then head to the full device table or compare two real devices.

All specifications are a public snapshot and may change — always confirm with the vendor's official site.