Online Ruler
A calibrated on-screen ruler for measurements. Run the browser-based check and follow its prompts to review the result.
The ruler starts from the browser's standard 96 CSS pixels per inch, which may not match your display's physical scale. Calibrate it before taking measurements.
Tool guide
About Online Ruler
Display a browser ruler in common screen units and calibrate it against a known physical measurement. Browser zoom, display scaling, and monitor pixel density affect the physical scale, so calibration matters.
How to use this tool
- 1Choose the unit and place a physical ruler beside the on-screen scale.
- 2Adjust calibration until a known length lines up with the screen ruler.
- 3Measure an object held against the display without pressing on or scratching the screen.
How the tool works
The page draws labeled CSS or calibrated units in the browser. CSS pixels are logical display units, not a guaranteed physical length; calibration maps them to a measured object for the current display and zoom setting.
Worked real-world example
If a known 5 cm reference spans 5 cm on the ruler after calibration, a nearby object can be compared at that same zoom and display setup.
Common scenarios and use cases
Make a rough size comparison when a physical ruler is not immediately available.
Important details and limitations
This is not a precision metrology instrument. Recalibrate after browser zoom, display scaling, or monitor changes, and do not use it for manufacturing, medical, or safety-critical dimensions.
Check the output unit as well as the number. If the result will be reused in another calculation, retain full precision where possible and round only at the end. For measurements governed by a technical specification, use the standard and tolerance required by that document.
Frequently asked questions
Why is calibration needed?
CSS pixels vary in physical size across displays and zoom settings.
Will calibration remain after changing monitors?
It may not; calibrate for the current setup.
Can I use it for exact engineering measurements?
No. Use a proper calibrated instrument.
About Online Ruler
A calibrated on-screen ruler for measurements. Run the browser-based check and follow its prompts to review the result.
How to use this tool
Use the controls above to run the check. Results are measured or calculated in your browser. Some device features require browser support, a secure connection, or permission from you.