Rulerly started from a simple frustration: most "online ruler" tools just guess at your screen's pixel density, which means what looks like a centimeter on screen often isn't actually a centimeter in real life. This leads to these sites being off by a wide margin, rendering them useless.
So Rulerly does the one thing that actually fixes that: it calibrates to your specific screen, using a physical reference almost everyone already has. Every bank/id/gift card is manufactured to the same standard size, so it works as a reliable, no-download way to teach the site exactly how big a pixel is on your device.
From there it's grown into a few more things I found genuinely useful while building it: a tracer mode for measuring a gap that isn't flush against an edge, the ability to move and resize the ruler anywhere on screen, and a rotate mode for lining up against a diagonal edge.
Rulerly is an independently built side project. It's free to use, supported by the small ads you'll see on the page, with no account, no download, and no tracking beyond standard site analytics.
Questions, bug reports, or ideas for what to add next:
contact@rulerly.net