Pixels Per Inch (PPI) Explained: Why Screens Don't Agree on Size

If you've ever compared a "life-size" image across a phone, a laptop, and a monitor, you've probably noticed it isn't actually the same size on all three. That's because of pixels per inch, or PPI.

What PPI actually means

PPI describes how many pixels are packed into one physical inch of a display. A phone screen might pack 400+ pixels into every inch, while an older desktop monitor might only fit 90–100. Since a "pixel" isn't a fixed physical size, an image that's exactly 200 pixels wide will look very different in real-world size depending on the PPI of the screen showing it.

Why browsers can't just know this

You'd think a website could simply ask the browser "how many physical inches is this screen," but there's no reliable web standard that exposes a device's true physical dimensions — browsers only know pixel counts, not physical size. That's the whole reason on-screen rulers need a calibration step: without knowing the real PPI, a ruler is just guessing.

How calibration fixes it

The most reliable fix doesn't require knowing your exact device model at all — it uses an object with a globally standardized size, like a credit card (85.6 mm wide everywhere in the world). By resizing an on-screen box until it matches a real card held up to the screen, a tool can back-calculate your screen's real PPI and scale everything else — rulers, comparisons, printable exports — correctly from that point on. That's the approach our online ruler uses.