Stop guessing the size of objects on your screen! Online Ruler is a professional, privacy-focused tool designed to turn your screen into a real-world measuring reference.
With our unique physical calibration technology, you can achieve 1:1 accuracy regardless of your monitor's resolution or scaling settings.
Align with standard objects like credit cards or A4 paper to calculate your screen's exact PPI.
Use as a clean, always-on-top independent window or as a transparent overlay on any webpage.
Customizable starting point (mm) and one-click reset to simulate real-world ruler edges.
Realistic wood grain design for better readability and a professional look.
Support for Metric (CM, MM) and Imperial (Inch, down to 1/16) units.
Drag any edge of the ruler to resize effortlessly without hunting for small handles.
All calibration data is stored locally in your browser. No data is ever uploaded.
Available in English, 中文, 日本語, 한국어, Deutsch, Français, and Español.
Click the extension icon and enter "Calibration Mode".
Select a reference (e.g., Credit Card) and adjust until the red box matches the physical object.
Open an independent window or add a ruler to the current page to start measuring.
Use the "Zero Offset" slider in the popup if you need to measure from a specific edge.
Click the puzzle icon 🧩 in the top right of your browser, find "Online Ruler", and click the pin icon next to it to fix it to the toolbar. Then simply click the icon to open the ruler anytime.
Yes. Since different screens have different resolutions, normal images aren't accurate. This extension uses "Physical Calibration"—by aligning it once with a credit card or A4 paper, it remembers your screen's actual size for 1:1 precision.
Yes. The system supports multiple reference objects, such as A4 paper or coins. We will add more standards like A5 paper or common phone screen sizes in the future.
The "Independent Window" pops up a clean, resizable ruler window that you can drag anywhere and keep on top of other apps. "Page Overlay" injects the ruler directly onto your current webpage with transparency support, so it won't block your reading.
You can use the "Zero Offset" feature. it simulates the protective end of a physical ruler, allowing you to set a starting gap (in mm) and reset it to zero with one click.
It's very simple. In overlay mode, you don't need to look for tiny corners; just drag anywhere along the ruler's edges to adjust its length.
No. Your calibration data (like PPI), unit preferences, and offsets are synced and saved in real-time. Settings apply instantly whether changed in the popup or across different pages.
Yes. This extension provides both Metric (CM, MM) and Imperial (Inch) scales, with high-precision imperial units divisible down to 1/16 inch.
No. All calibration data and preferences are saved locally in your browser. No data is ever uploaded to servers.
It uses SVG technology to dynamically generate a realistic wood visual effect. It's beautiful, clear, and very lightweight, so it won't slow down your browser at all.
Initial release.
This extension is open source and the code is available on GitHub. Contributions are welcome!
View Source Code on GitHub