Why I Built an Online Ruler That Needs Calibration

Most "online ruler" pages look simple: draw a rectangle, add centimeter ticks, add inch ticks, done. That works only if every screen maps CSS pixels to real-world size in the same way. They do not. A 13-inch laptop, a 27-inch external monitor, a phone, a tablet, and a 4K display can all render the same CSS width with different physical sizes. Browser zoom, operating-system scaling, Retina-style display modes, and external monitors all make the problem worse. So I built a small browser-based rule