PPI calculator
Find your screen’s pixels per inch, then apply it to the ruler
What is PPI?
PPI (pixels per inch) is the number of pixels that fit into one physical inch of your display. It is calculated by dividing the screen’s diagonal resolution in pixels by its diagonal size in inches: PPI = √(width² + height²) ÷ diagonal. Browsers assume 96 PPI by default, but real displays range from about 80 to over 400 PPI, which is why an uncalibrated on-screen ruler is never accurate.
You can also find your PPI without math: measure the diagonal of your visible screen area with a tape measure, enter it above, and the calculator does the rest. Thenopen the online ruler - it reads the PPI you save and draws the scale to actual size.
Common PPI values
- 24″ 1920×1080 monitor: 91.8 PPI
- 27″ 2560×1440 monitor: 108.8 PPI
- 13.3″ 2560×1600 laptop: 227 PPI
- 6.1″ 2532×1170 phone: 460 PPI
- A credit card is 85.6 mm wide (ISO/IEC 7810 ID-1) - the easiest real-world calibration object.