What PPI actually measures
Pixels per inch counts how many individual pixels fit across one inch of a display, measured along its diagonal. A screen with a higher PPI packs its pixels closer together, which is why the same photo looks sharper on a phone than blown up on a projector: the pixel count might be similar, but the phone squeezes them into a much smaller physical space.
It is a physical property of the screen, fixed by its resolution and its size, and it has nothing to do with how a web browser lays out a page. That second part is where most on-screen rulers go wrong, and it is worth separating clearly from the first.
How to calculate it
PPI comes from the resolution and the diagonal size, using the screen's diagonal pixel count divided by its diagonal size in inches:
PPI = √(width² + height²) ÷ diagonal size in inches
A 24-inch monitor at 1920×1080 has a diagonal pixel count of about 2203, so 2203 ÷ 24 comes out to roughly 92 PPI. Move the same 1920×1080 resolution to a 15.6-inch laptop panel and the calculation gives about 141 PPI, nearly 50% denser, from identical pixel counts on a smaller sheet of glass. Resolution alone never tells you how sharp a screen is; size has to come with it.
Why the spec sheet number is not what your browser sees
A web browser does not lay pages out in physical pixels. It uses the CSS pixel, which the W3C defines as a fixed 1/96 of an inch, a convention chosen in the 1990s and never revisited even as real screens got far denser than 96 PPI. A browser translates that fixed unit into your screen's real pixels using a scale factor, most often 1x on an ordinary monitor and 2x or 3x on a phone or a Retina-class laptop display.
The problem for any ruler drawn with plain CSS is that this scale factor is reported by the operating system, and the report is frequently wrong for the purpose of physical measurement. Display scaling set to anything other than 100%, an external monitor plugged into a laptop, or simply a device the browser does not recognise, and the figure it works from stops matching the glass in front of you. A tool that trusts it alone can be off by a third or more.
Typical PPI across common devices
| Device | Typical resolution | Approx. PPI |
|---|---|---|
| 24" monitor, 1080p | 1920×1080 | ~92 |
| 27" monitor, 1440p | 2560×1440 | ~109 |
| 27" monitor, 4K | 3840×2160 | ~163 |
| 13" laptop, Retina-class | 2560×1600 | ~227 |
| Modern smartphone | ~1080×2400 | ~390–460 |
The pattern that matters is not any single row, it is the spread: a factor of nearly five between a large monitor and a phone, all showing pages built with the same fixed-width CSS pixel.
What this means for anything you measure on screen
A ruler, ring sizer or paper size viewer that only reads your screen's reported resolution is making the same mistake as reading a spec sheet and assuming it applies: correct for one class of device, wrong for most others, and silently wrong rather than visibly wrong. The fix is not a better lookup table of device models, since new devices ship faster than any table can track and external monitors defeat it entirely. It is to measure the screen directly.
That is what calibrating against a bank card does. Every payment card in the world is built to the same ISO/IEC 7810 ID-1 standard, 85.6 mm wide, so matching an on-screen outline to a real card reveals the screen's actual pixel density in one step, without needing to know its model, its resolution setting or its operating system. The Screen Size Checker does exactly this, and the same calibration then carries over to every other tool on this site.