Screen Ruler
A ruler in real units — calibrated with a bank card, because a browser pixel is a fiction and an uncalibrated on-screen ruler is simply wrong.
Ruler
Calibrate with a bank card
Hold any bank, credit or ID card against the screen and drag the slider until the outline is exactly the same width. Every card in the world is 85.6 mm wide — it is an international standard — which makes it the most reliable ruler most people own.