Online Ruler in CM, MM, and Inches

This free online ruler lets you measure small objects on your phone, tablet, laptop, or desktop screen. Use the centimeter and millimeter scale for metric measurements, or switch to inches for inch-based measurements. Every tick is drawn to real-world proportions, so once the ruler is calibrated you can read exact lengths without a physical measuring tape.

How to Use the Online Ruler

• Choose centimeters and millimeters, or switch to inches.
• Calibrate the ruler for your screen using your screen diagonal or a credit card.
• Place the object against the screen and drag the guide to read the measurement.
• Use fullscreen mode or rotate the ruler if you need more measuring space.

Why Calibration Matters

Screens have different pixel density, zoom settings, and physical sizes, so a ruler that looks right on one device can be off on another. Calibration helps the virtual ruler match real-world size. For a quick setup, enter your screen diagonal in inches, or match the on-screen guide to a standard credit card, which is exactly 85.60 mm wide. Your calibration is saved in your browser for next time.

Measure on Phone, iPhone, Tablet, or Desktop

The ruler works in modern browsers on both mobile and desktop devices. On phones, including iPhone and Android, rotate the ruler or switch to fullscreen mode for easier measuring. Because it runs entirely in the browser, you can use the ruler on your phone anywhere — no app to install and nothing to update.

Frequently Asked Questions

• Is this online ruler actual size? It can be actual size after calibration. Without calibration it is only an estimate, because every screen has a different pixel density. Calibrate with your screen diagonal or a credit card to get a true actual-size ruler.
• Can I use this ruler in inches? Yes. Switch from CM/MM to inches at the top of the ruler. The inch scale shows whole inches down to sixteenths for precise imperial measurements.
• Can I use it on my phone? Yes. It works on phone screens, including iPhone and Android devices. Rotate the ruler or use fullscreen mode to measure longer objects comfortably on a small screen.
• Do I need to install an app? No. The ruler works directly in your browser. There is nothing to download, install, or sign up for.
• How do I calibrate the ruler with a credit card? Choose the credit card method, then resize the on-screen card outline until it matches a real card held against the screen. A standard ID-1 card is 85.60 mm wide, and the tool uses that to set real-size accuracy.