How this PPI calculator works
PPI — pixels per inch — measures how densely the pixels of a display are packed into its physical surface. A screen with more pixels squeezed into each inch shows finer detail and smoother edges. Because the pixels of a rectangular screen run in two directions, you cannot simply add the horizontal and vertical counts; you have to find the length of the diagonal in pixels first.
That diagonal comes from the Pythagorean theorem: the pixel diagonal equals the square root of the horizontal resolution squared plus the vertical resolution squared. Dividing that pixel diagonal by the screen's diagonal size in inches gives the PPI. From the PPI the tool also derives the dot pitch (25.4 ÷ PPI, in millimetres), the total megapixels (width × height ÷ 1,000,000), the aspect ratio simplified with the greatest common divisor, and the physical width and height in inches.
Common screens land at predictable densities, so the table below is handy for sanity-checking a purchase or comparing two panels of different sizes.
| Screen | Resolution | Diagonal | ≈ PPI |
|---|---|---|---|
| 1080p monitor | 1920×1080 | 24 in | 92 |
| 1440p monitor | 2560×1440 | 27 in | 109 |
| 4K monitor | 3840×2160 | 27 in | 163 |
| Smartphone | 1080×2340 | 6.1 in | 420 |
Reference note: results assume the resolution describes square pixels on a flat panel and that the diagonal is the active display area. Marketing figures sometimes round the diagonal, so a real panel may differ from the calculated value by a fraction of a PPI.
Frequently asked questions
- What is PPI / pixels per inch?
- PPI is the number of pixels packed into one inch of a display. A higher PPI means smaller, denser pixels and a sharper image, because more detail fits into the same physical space.
- How do I calculate PPI?
- Take the square root of the horizontal resolution squared plus the vertical resolution squared to get the pixel diagonal, then divide by the diagonal screen size in inches. A 1920×1080 screen at 24 inches works out to roughly 92 PPI.
- What is a good PPI for a monitor / phone?
- It depends on viewing distance. Desktop monitors look crisp at about 90–160 PPI, laptops around 120–220 PPI, and phones often exceed 400 PPI. Past roughly 300 PPI at phone distance most people cannot see individual pixels.
- What is dot pitch?
- Dot pitch is the physical distance between the centres of neighbouring pixels, in millimetres. It is the inverse of density: dot pitch = 25.4 ÷ PPI. Lower dot pitch means a finer display.
- How do I find a screen's aspect ratio?
- Divide both resolution figures by their greatest common divisor. 1920 and 1080 share a GCD of 120, so 1920×1080 simplifies to 16:9. The helper above does this for any width:height.
- What's the difference between PPI and resolution?
- Resolution is the total pixel count, such as 1920×1080. PPI is how tightly those pixels are packed into the physical screen. The same resolution looks sharp on a small panel but coarse on a large one.