online ruler ppi calibration
A CSS pixel has no guaranteed physical size, so this ruler asks for an object of known dimension on screen to calculate your monitor real PPI before showing any measurement in centimeters or inches.
The math is 323 divided by 85.60, giving pixels per millimeter, times 25.4 millimeters per inch, landing at approximately 95.8 pixels per inch.
Screens with a different operating system scale factor need recalibration, because the browser reports CSS pixels already adjusted by the scale factor, not the panel raw physical pixels.
Assuming 96 PPI, the CSS historical default value, ignores the panel real density, so the displayed measurement diverges from the physical measurement on high- or low-density screens.
No. Once calibrated it is close to real for estimates, but accuracy depends on calibration, zoom and system scaling. For technical work, use a certified instrument.
Because a CSS pixel has no guaranteed physical size; a bank card in the ISO/IEC 7810 ID-1 standard always measures 85.60 mm wide, so adjusting the bar to match the card calculates your screen real PPI instead of assuming a generic value.
Yes: the ISO/IEC 7810 ID-1 standard fixes 85.60 mm by 53.98 mm for credit and debit cards from any bank or network, and also for most ID cards and driver licenses.
Divide the card width in pixels by its real width of 85.60 mm, multiply by 25.4 mm per inch; a 323-pixel card gives 323 ÷ 85.60 × 25.4, approximately 95.8 pixels per inch.
Yes: switching monitors changes the panel real pixel density, and changing browser zoom changes how many CSS pixels fit on screen; both cases require measuring the card again to get the correct PPI for that moment.
Calibration stays only in this tab state.