Online ruler

Match the reference line to a known length before measuring anything on your screen.

On your device · No account

Privacy details

Online ruler processes your input in this browser. Inputs and results are excluded from analytics. Recent tools save tool names, visit counts and last-visit times. Saved tools store only their names. Draft saving is optional and stays on this device; delete saved text with the draft controls. Loading text from a URL is optional and contacts that server; its URL and your IP address are visible to the server. An optional “Use in” action keeps a handoff in this tab. The next tool removes it on arrival and ignores it if more than a minute has passed.

Copy link includes the inputs you choose to share in the address. Anyone with that link can reopen them. Files and generated results are excluded. Password and key tools share settings only.

Known reference length (mm)

Reference line width (screen pixels)

Match this line to your known reference.

Your result

Calibration not confirmed.

More options and calculation details

Adjust the pixel width until the blue line matches a physical ruler or an object whose length you measured. Confirm calibration. Zooming, resizing or moving to another display requires calibration again.

Reset reloads the tool with its starting values and releases open files and device controls. Saved drafts stay on this device until you delete them.

Links include your inputs and settings. Anyone with the link can see them, so leave out private text. Files and generated results are left out; random draws run again.

Keyboard shortcuts

Updated 2026-09-19

How this conversion works

A browser cannot reliably know the physical size of a screen pixel. Calibration divides the displayed reference width by a known physical length.

After matching the reference, the ruler marks millimeters using that ratio. Its scale stays local to this tab and is never saved as a universal screen measurement.

Next: Length converter

A worked example

If 200 screen pixels line up with a measured 50 mm reference, the calibrated scale is 4 screen pixels per millimeter.

Next: DPI and PPI calculator

Before you use the result

Accuracy depends on your reference and alignment. Recalibrate after zoom, display or window changes. Do not use this for precision work or safety-critical dimensions. Nonzero numeric inputs for these compound calculations are limited to 1e-40 through 1e40.

Next: Paper size converter

Cite this page

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ToolOctopus. “Online ruler.” Updated 2026-09-19. https://tooloctopus.com/online-ruler.

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