An online ruler (cm/mm) is a digital tool or website that allows you to measure distances, lengths, or objects using centimeters (cm) or millimeters (mm) through your internet browser. It provides a virtual ruler on your screen, which you can use to accurately measure various items or lengths without the need for a physical ruler or measuring tape. Online rulers are particularly helpful when you don't have access to traditional measuring tools or are working in a digital environment.

Here's how you can use an online ruler (cm/mm):

1. Open a web browser: Launch your preferred web browser on your computer or device.

2. Search for an online ruler: Use a search engine to find an online ruler tool or website. You can search for terms like "online ruler cm," "virtual ruler mm," or similar variations.

3. Choose an online ruler website: Select a reliable and user-friendly online ruler website from the search results. Make sure to choose a trustworthy site to ensure accurate measurements.

4. Set the scale units: Once you access the online ruler website, you may need to specify the scale units you want to work with, such as centimeters (cm) or millimeters (mm). Select the appropriate option.

5. Calibrate the ruler (if applicable): Some online rulers may require calibration to ensure accuracy. This involves comparing the on-screen ruler to a known physical measurement. Follow the instructions provided by the website to calibrate the ruler, if necessary.

6. Measure objects or distances: With the online ruler displayed on your screen, you can now start measuring various items or distances. Align the digital ruler with the object or length you wish to measure, and read the measurement value displayed on the screen. Move the ruler along the object to measure different sections or lengths.

7. Adjust zoom or size (if needed): Depending on the online ruler website, you may have options to adjust the zoom level or size of the ruler on your screen. Utilize these controls to ensure a clear and accurate measurement.

8. Take screenshots or notes (if desired): If you need to record or save the measurements, consider taking screenshots of the on-screen ruler along with the objects you measured. You can also write down the measurements in a separate document or note-taking application.

9. Exit the online ruler: Once you have completed the necessary measurements, you can close the online ruler website by closing the browser tab or navigating to a different website.

Please note that the accuracy of an online ruler depends on several factors, including your screen resolution, zoom level, and the calibration (if applicable). As a result, online rulers may not be as precise as physical measuring tools. Nonetheless, they can provide a suitable estimation for many applications.