I built an online ruler that calibrates to your actual screen I wanted a simple way to measure small physical objects directly against my screen, but there's a fundamental problem with online rulers: every screen has a different pixel density, so a ruler rendered using a generic scale won't necessarily match real-world dimensions. So I built Real Size Online Ruler to solve that. The part I focused on most is the calibration . A lot of online rulers offer credit-card calibration, but I found that the calibration interaction itself can be surprisingly imprecise. If you're placing a physical card against an on-screen placeholder, the borders need to give you a clear reference for exactly where the card edges should align. So I made the credit-card calibration work by placing your