How to use the online ruler
Use this screen ruler when you need a quick measurement and do not have a physical ruler nearby. Switch between centimeters and inches, and match a small object to the scale. Calibrate your display before relying on the reading as a physical measurement.
Why screen centimeters need calibration
A browser pixel is not a fixed physical length on your monitor. Pixel density, operating-system scaling and browser zoom can all change the size of the ruler. The initial scale is a starting point, so calibrate on the display you are actually using.
Calibrate with a payment card
The long edge of a standard payment card is 8.56 cm. Open screen calibration and adjust the reference line to match that edge. Align the total width rather than the rounded corners. Save the scale, and repeat calibration whenever you change your display or browser zoom.
Read centimeters and inches
In centimeter mode, numbered divisions are centimeters and the smallest divisions are millimeters. Inch mode divides each inch into 16 parts. One inch equals exactly 2.54 centimeters. Align one end of your object with zero and read the value at the other end.
Continue beyond the screen with marks
Calibrate first, align the end handle with the end of your object or a continuation point, and choose “Mark & continue.” The section is saved and the endpoint returns to the left start guide. Move that same point on your object to the left guide and measure the next section. Total length includes saved sections plus the current section. “Use right edge” moves the endpoint to the usable edge; “Undo mark” restores the previous section. Avoid gaps or overlaps when repositioning the object.
A few common questions
Does this work on a phone?
Yes. Mobile browsers are supported. Each section is limited by the screen, and marking lets you continue after repositioning the object.
Do I need to calibrate every time?
Your calibration is saved in this browser when storage is available. Calibrate again after changing displays, devices or browser zoom.