📋 How to Use This Ruler
- Calibrate your screen: click the Calibrate Screen button and drag the blue box to match a physical credit card (85.6 mm / 8.56 cm). This only needs to be done once per device.
- Place your object: align the left edge against the red zero line at the ruler's left edge.
- Drag the green marker: click or tap at the object's right edge, or drag the green line to that point. The measurement displays in real time.
- Switch units if needed: toggle between CM, MM, and inches with the unit buttons above the ruler.
- Scroll for longer objects: scroll or swipe right to extend the ruler up to 50 cm / 20 inches.
🎯 Accuracy & Calibration
After calibrating with a credit card, this ruler achieves approximately ±0.5 mm accuracy on most screens. High-DPI and Retina displays can achieve ±0.3 mm or better.
Accuracy depends on how precisely you matched the calibration box to your credit card, your screen's pixel density (higher PPI = more accurate), whether your browser is at 100% zoom, and whether the object is laid flat on the screen.
📱 Works on Any Device
This ruler works equally well on smartphones, tablets, laptops, and desktop monitors. Each device stores its own calibration, so you can calibrate your phone and your laptop separately and both will show accurate actual-size measurements.
On touch screens, tap the ruler to place the measurement marker. On desktop, click or drag. The ruler is optimized for fast, smooth interaction on all device types.