Pixel360PPI formula
A PPI calculator measures pixel density: how many pixels fit into one inch of screen. Because screens are sold by their diagonal, you first find the number of pixels along the diagonal with the Pythagorean theorem, then divide by the diagonal length.
diagonal pixels = √(width² + height²)
PPI = diagonal pixels ÷ diagonal inches
dot pitch (mm) = 25.4 ÷ PPI
Worked examples
- 24-inch 1920×1080: √(1920² + 1080²) = 2202.9 → 2202.9 ÷ 24 = 91.8 PPI, pitch 0.277mm
- 27-inch 3840×2160: √(3840² + 2160²) = 4405.8 → 4405.8 ÷ 27 = 163.2 PPI, pitch 0.156mm
- 6.5-inch 1080×2400 phone: √(1080² + 2400²) = 2631.8 → 2631.8 ÷ 6.5 = 404.9 PPI, pitch 0.063mm
Notice that the 27-inch 4K screen has the same resolution as a 55-inch 4K TV but about twice the density, because the same pixels are packed into roughly half the diagonal.
Pixel density of common screen sizes
Calculated with the formula above from resolution and diagonal size. Real panels can differ slightly because the visible diagonal is often a fraction of an inch off the advertised size.
| Resolution | Diagonal | Type | PPI | Dot pitch |
|---|---|---|---|---|
| 1920×1080 | 13.3" | Small laptop | 165.6 | 0.153mm |
| 1920×1080 | 15.6" | Laptop | 141.2 | 0.180mm |
| 1920×1080 | 24" | Desktop monitor | 91.8 | 0.277mm |
| 2560×1440 | 27" | Desktop monitor | 108.8 | 0.233mm |
| 3440×1440 | 34" | Ultrawide monitor | 109.7 | 0.232mm |
| 3840×2160 | 27" | Desktop monitor | 163.2 | 0.156mm |
| 3840×2160 | 32" | Desktop monitor | 137.7 | 0.184mm |
| 5120×2880 | 27" | Desktop monitor | 217.6 | 0.117mm |
| 3840×2160 | 55" | TV | 80.1 | 0.317mm |
| 1080×2400 | 6.5" | Phone | 404.9 | 0.063mm |
Why PPI matters for designers
PPI explains why the same design looks crisp on one screen and soft on another. Operating systems compensate for high density with scaling: a 163 PPI monitor is usually run at 150% so that interface elements are a readable physical size. That is why CSS pixels are not the same as hardware pixels. A screen with a device pixel ratio of 2 uses four physical pixels for every CSS pixel, and images need twice the CSS dimensions to look sharp. The image resize calculator helps you work out those 2× export sizes, and the px to rem converter keeps type scalable across densities.
Viewing distance matters too. A TV at 80 PPI looks sharp from across a room, while a phone at the same density would look pixelated at arm’s length. Higher PPI only helps up to the point where your eye can no longer separate individual pixels at your usual distance. For background on how density is defined and measured, see the pixel density article on Wikipedia.
Common mistakes
- Adding width and height instead of using the diagonal. PPI is per inch along any direction, so you need the diagonal pixel count, not the sum.
- Mixing centimeters and inches. The formula uses inches. Divide a diagonal in centimeters by 2.54 first.
- Using scaled resolution. Enter the panel’s native resolution, not the “looks like” resolution set in display settings.
- Confusing PPI with print DPI. Screen density and print resolution are separate questions. For prints, use the DPI calculator.
Frequently asked questions
What is a good PPI for a monitor?
At typical desk distances, around 90–110 PPI looks fine at 100% scaling, and 140 PPI or more looks noticeably sharper, especially for text. Higher-density screens are usually run with 150% or 200% scaling so text stays a comfortable size.
Is PPI the same as DPI?
Not strictly. PPI (pixels per inch) describes the pixel density of a screen or a digital image. DPI (dots per inch) describes how many ink dots a printer lays down. The terms are often used interchangeably, especially when talking about image resolution for print.
Why does my 4K TV have lower PPI than my phone?
PPI depends on screen size as well as resolution. A 55-inch 4K TV spreads 3840×2160 pixels over a large area (about 80 PPI), while a phone fits a similar number of pixels into about 6 inches, giving 400 PPI or more.
What is dot pitch?
Dot pitch (or pixel pitch) is the distance from the center of one pixel to the next, usually given in millimeters. It is 25.4 ÷ PPI, so a 109 PPI monitor has a pitch of about 0.233mm. Smaller pitch means a finer, sharper image.
Does the PPI calculation work for curved or ultrawide screens?
Yes. The formula only needs the resolution and the diagonal measurement. For curved screens, manufacturers quote the diagonal of the flat panel, which is what the formula expects.