ON Ruler is a free measuring app that provides a quick and accurate way to measure small objects with your Android device. With two important ruler modes - 2-Lines Ruler and 4-Lines Ruler - you can easily measure the dimensions of objects anytime and anywhere. The app also lets you switch between millimeter and inch units with just one click.

The app is simple and smart, and it automatically displays the width, length, and area of objects on your screen. You can measure any object that fits within your screen size by placing it on your device's screen and moving the line limiters. For larger objects, you can use ON Measuring Tape, ON-2D Camera Measure, or ON-3D Camera Measure, all of which are free measuring tools. Calibration is required only once per installation for perfect accuracy.

ON Ruler also features customizable themes, ruler line colors, text colors, and text sizes. You can also lock or unlock the screen ruler and keep your screen awake.

Overall, ON Ruler is a helpful measuring tool that combines convenience and accuracy in one app.