Online Screen Ruler – Calibrated Millimeter & Inch Ruler

How to Use the On-Screen Ruler

1. Calibrate for exact size: The ruler works immediately at an approximate scale. For true 1:1 accuracy, click "Calibrate Now" and place an ATM, credit, or debit card over the screen. Align the card edge to the outline to configure exact pixel scaling.
2. Place the Object: Place your coin, key, screw, or card flat against the device display. Align one edge with the zero mark (or left jaw).
3. Drag the Measurement Jaws: Touch or click-and-drag the orange pointer to span the object's width. The readout displays the exact measurement instantly.
4. Switch Units: Toggle between mm (millimeters), cm (centimeters), and in (inches).

Accuracy and Optical Precautions

Screen rulers are accurate to approximately ±0.5 mm when calibrated correctly. Keep in mind:

• Protective Glass: Thick screen protectors introduce a small optical gap (parallax). Always view straight from above.
• Browser Zoom: Keep your browser zoom at 100%. If you pinch-zoom or adjust browser scale, press "Calibrate" to reset the scaling factor.
• For in-depth details on pixel density and accuracy boundaries, read our Screen Ruler Calibration Guide.