Why screen calibration matters
CSS defines an inch as 96 browser pixels, but physical pixel density and operating-system scaling vary. Matching a known 85.60 mm card gives this browser a device-specific scale for more realistic measurements.
Calibrated Screen Tool
Free Online Ruler
An online ruler is an on-screen scale for measuring small objects in inches, centimeters, or millimeters. Calibrate this ruler to your display, then drag the guide to read an actual-size measurement.
Measure on your screen
Calibrate first for the best actual-size result.
Measurement
Scale status
Default browser scale
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7.62 cm
cm scaleAccuracy depends on calibration and browser zoom. Keep zoom at 100%, recalibrate after changing displays, and use a physical ruler for precision-critical work.
Quick start
How to use the online ruler
- 1
Calibrate the display
Match the on-screen card to a physical bank card and save the scale.
- 2
Choose a ruler
Select inches, centimeters, or millimeters and horizontal or vertical mode.
- 3
Move the guide
Drag or use the measurement slider to align the cyan guide with the object edge.
- 4
Read the result
Use the live measurement shown beside the ruler and verify calibration when needed.
Online ruler FAQ
Is an online ruler actual size?
An online ruler can show actual-size measurements after calibration. Browser pixels vary across screens, so match the calibration card to a physical bank card before measuring when accuracy matters.
How do I calibrate the screen ruler?
Place a standard credit or debit card against the screen, adjust the on-screen card until both widths match, then save. A standard ISO ID-1 bank card is 85.60 millimeters wide.
Can I measure in inches and centimeters?
Yes. Switch between inches, centimeters, and millimeters. The ruler and draggable guide update immediately while preserving the same physical position.
Does calibration leave my device?
No. The calibration scale is stored only in your browser on this device. The tool does not upload measurements or device information.
Is a screen ruler suitable for precision work?
Use it for quick estimates, classroom activities, and small desk objects. For manufacturing, health, or safety-critical measurements, use a verified physical measuring instrument.
Related free tools
Measure, design, and build with Zap Code.