Auto-detect device
Best for common phones, tablets, and laptop displays. The site estimates screen density from device signals and confirms the match before saving it.
CM / MM / INCH
Online ruler that measures actual size.
Choose the screen edges you want, calibrate once, and measure in centimeters, millimeters, or inches.
Current scale
Default CSS scale. Calibrate for actual size.
Move pointer over the ruler.
Live measurement
0 cm
Measure from the nearest visible edge. Calibration is saved on this device.
Three ways to calibrate.
Use the quickest method that fits your screen, then switch back to the ruler.
Method 1
Auto-detect device
The detector checks common screen classes and applies the closest physical scale it can infer.
No device detection has been applied yet.
Method 2
Screen diagonal
Enter the display diagonal in inches. This is usually listed in laptop, monitor, phone, or tablet specs.
Method 3
Credit card calibration
Place a standard card over the guide and move the slider until the on-screen card matches its physical width.
Standard card size: 85.60 mm by 53.98 mm.
85.60 mm
Built for real measuring, not just a picture of a ruler.
The scale is drawn from your saved pixels-per-millimeter value, so every selected edge updates after calibration.
Screen diagonal
Best for monitors and laptops. Enter the display diagonal printed in your device specs and the ruler converts screen pixels into physical units.
Credit card calibration
Best when accuracy matters. Match the on-screen guide to a standard card width of 85.60 mm and save the measured scale.
Use it for quick desk measurements.
Simple Online Ruler is useful for checking small objects, paper margins, craft cuts, screen mockups, packaging sizes, and school measurements when a physical ruler is not nearby.
How to use the online ruler
01
Pick ruler edges
Use top, right, bottom, left, or multiple edges depending on the object position.
02
Calibrate once
Auto-detect, enter your screen diagonal, or match the credit-card guide.
03
Measure at 100%
Keep browser zoom unchanged and place the object against a selected edge.
Online ruler FAQ
How accurate is an online ruler? +
An online ruler can be accurate after calibration. Credit card calibration is usually the most dependable because it uses a physical 85.60 mm reference object on your own screen.
How do I calibrate the ruler in centimeters? +
Open calibration, choose screen diagonal or credit card calibration, apply the result, then keep browser zoom at 100%. The saved scale is used for centimeter, millimeter, and inch ticks.
Can I use this as a millimeter ruler? +
Yes. Select mm in the unit control. The ruler still shows major centimeter marks, but the measurement readout and tick spacing are based on millimeters.
Why does browser zoom affect an actual-size ruler? +
Browser zoom changes the size of CSS pixels on the screen. A real-size ruler should be calibrated and used at 100% zoom on the same display.
What credit card size is used for calibration? +
The calibration guide uses the ISO/IEC 7810 ID-1 card size: 85.60 mm wide by 53.98 mm tall.