Rulerly

What is Rulerly?

Most "online ruler" tools you'll find just guess at your screen's pixel density and hope for the best, and they're often extremely inaccurate. Rulerly fixes that with a 15-second calibration using any card in your wallet (they're all the same size), so what you see on screen is actually true to scale. In addition it features tracing tools, moving/resizing features, diagonal mode, and it works on any device.