Online Ruler in Actual Size for CM, MM & Inches

An online ruler displays measurement marks on a screen so you can compare a small physical object with centimetres, millimetres or inches. Physical accuracy depends on correct screen calibration.

Yes, after calibration. A browser cannot reliably determine the physical size of every screen, so the ruler must be matched to a known object, calculated from trusted display specifications or configured with a known CSS pixels-per-inch value.

CSS pixels, operating-system scaling, browser zoom and display density vary. Calibration establishes how many CSS pixels on the current screen correspond to one physical inch.

Not reliably. The browser can report CSS screen dimensions and device-pixel ratio, but those values do not by themselves reveal the display's physical width, height or true pixel density.

Device pixel ratio describes how many device pixels are used for one CSS pixel. It helps render sharp graphics but is not a complete measurement of the screen's physical PPI.

Choose the standard payment or ID card reference, place the card against the calibration bar, resize the bar until its endpoints match the card's 85.60 mm width, and press Apply Reference Calibration.

Yes. Choose Custom known length, enter the object's verified length in millimetres, resize the bar to match it and apply the calibration.

Enter the display diagonal in inches and its native horizontal and vertical pixel resolution. The tool calculates physical PPI and divides it by the entered device-pixel ratio to estimate the required CSS pixels per inch.

It is the number of CSS layout pixels that should occupy one physical inch on the current display at the current browser zoom. Use it when you already know a tested calibration value.

CSS defines one inch as 96 CSS pixels for layout calculations. On many screens that CSS inch is not exactly one physical inch, so the baseline is clearly marked uncalibrated.

Yes. Changing browser zoom can change the physical size of CSS pixels. Keep zoom unchanged after calibration or recalibrate when the device-pixel ratio warning appears.

Usually yes. Different monitors can have different physical pixel density and operating-system scaling, even when their reported resolutions look similar.

Yes. Calibrate the phone in its normal orientation and browser zoom, then use the horizontal or vertical ruler. Fullscreen availability and browser controls vary by mobile browser.

Yes. Tablets often provide a larger measuring surface, but they still require calibration because CSS and device pixels do not directly reveal physical screen dimensions.

The ruler supports combined centimetre and inch marks, centimetres with millimetre subdivisions, millimetre labels, and inches divided into eighths or sixteenths.

Yes. Choose Vertical ruler. The same calibrated physical scale is drawn from top to bottom.

Dual mode draws horizontal and vertical calibrated scales in one square workspace so you can compare width and height directions. Very long dual rulers may require scrolling.

The orange start and end guides mark two positions on the scale. Drag either guide on the ruler or enter positions in millimetres, centimetres or inches to calculate the distance between them.

Yes. Choose free movement, one-millimetre, five-millimetre or one-sixteenth-inch snapping.

Accuracy is limited by calibration quality, object alignment, screen flatness, browser zoom, display scaling and the precision of the reference object. Verify important measurements with a physical measuring instrument.

Yes. After calibration, compare several known lengths across the screen. Recalibrate if the error grows or if the browser, zoom level, display or orientation changes.

Yes when the browser permits it. Fullscreen requires a direct user action and can be denied. The tool uses a full-window focus mode when a fullscreen request is unavailable or rejected.

The browser or embedded page may restrict fullscreen, or the request may not follow a direct user action. Focus mode remains available as a fallback.

PPI usually describes display pixels per inch, while DPI traditionally describes printed dots per inch. Online ruler calibration concerns how CSS pixels map to physical inches on a display.

The ruler keeps its calibrated scale rather than shrinking to fit. Longer rulers use an internal scrolling measurement surface, so you can inspect sections without changing the physical scale.

A real-size ruler can be wider or taller than the available viewport. The internal scrollbar prevents page overflow while preserving the calibrated physical size.

It changes the selected ruler length to approximately fill the visible ruler area. It does not stretch or compress the calibrated scale.

Yes. The tool can generate a vector ruler using the current orientation, length, units and guide positions, plus a text calibration report.

It can contain physical width and height units, but print dialogs may scale pages. Print at 100 percent or Actual Size, disable Fit to Page and verify the output against a physical ruler.

No page-specific request sends ruler settings, guide positions or calibration values to a server. The calculations and generated files remain in the current browser page.

Only when you select Remember calibration on this device. That optional setting stores the calibration value in this browser. Clear Stored Calibration removes it.

Return browser zoom to the level used during calibration, confirm the window is on the same display, check the selected reference length, recalibrate and compare the result with more than one known physical length.