Calibrate the on-screen ruler before you trust it
How to use it
The ruler is a scale drawn on the screen. Screens differ in pixels per inch, zoom, and operating-system scaling, so an uncalibrated ruler is only a picture of centimeters. Hold a physical ruler you trust against a known length, match the on-screen scale, and only then estimate. It is not a certified measuring instrument and not a substitute for a tape on a workpiece. Nothing is uploaded because there is no file. The estimate stays in the browser. If you change zoom or move the window to another display, calibrate again.
The file stays in this browser. NexTool does not upload it and does not keep a copy after you leave the page. Download the result and check it before you send it on. If the tab runs out of memory, the job stops and the original is still the one on your computer.