Online Ruler

Place a small object beside the on-screen ruler, align one end with zero, and drag the marker to the other end. The readout shows the length in millimetres, centimetres and decimal inches.

Choose Horizontal for width or Vertical for height. Fullscreen gives the ruler more space, but does not improve its physical accuracy. For a measurement you intend to compare with a real ruler, calibrate the screen first.

A browser lays out pages in CSS pixels, which do not reliably correspond to physical millimetres on every display. Until calibrated, this ruler uses the CSS reference of 96 pixels per inch. That is an approximate on-screen scale, not a measurement of your device's actual pixel density.

Select Calibrate and match the on-screen guide to the straight edge of a standard ID-1 bank or identification card: 53.98 mm for the short edge or 85.60 mm for the long edge. Use a flat, undamaged card, view the guide straight on, and avoid pressing on the screen.

You can also calibrate with an object of known length, or enter your screen's diagonal size in inches. Card or known-length matching provides a direct physical reference; the screen-size option depends on the device dimensions reported by the browser. Check any calibrated result against a physical ruler if accuracy matters.

An online scale can refer to the centimetre, millimetre or inch markings used to measure length on a screen. This tool provides that kind of length scale; it does not weigh objects or calculate map or drawing scales.

The metric and inch markings share one draggable measurement marker. Calibrate the ruler before treating those markings as actual physical size.

The metric scale is divided into millimetres, with centimetre labels. The inch scale has marks at sixteenths of an inch, while the live measurement is shown in decimal inches. 10 mm = 1 cm, and 25.4 mm = 1 inch.

Use the ruler to measure a label, card, photograph or small craft piece, keeping the object aligned with the zero mark.

The ruler works in mobile and desktop browsers. Switch to vertical mode for upright measurements; use fullscreen when the ordinary page leaves too little room. You can drag the marker with touch or a mouse. When it has keyboard focus, use the arrow keys for small adjustments.

The approximate marker moves in 1 mm steps; the calibrated marker supports 0.5 mm steps. Those steps describe the readout, not a guarantee of half-millimetre physical accuracy. Avoid measuring sharp, hot, wet, heavy or abrasive objects against a screen.

Screen density, operating-system scaling, browser zoom and a change of monitor can alter the physical length represented by CSS pixels. Olivez checks the saved calibration against screen dimensions, device pixel ratio and viewport scale, and can ask you to recalibrate when those values change.

These checks cannot identify every display change. If the scale looks wrong—or you move the browser to another monitor—calibrate again. For dimensions that must be dependable, use a physical ruler or calliper.