Most online rulers assume every screen has 96 pixels per inch, so they're the wrong size on almost every device. This one works out which screen you have (iPhone, Android, iPad, Mac or Windows laptop) and uses its real pixel density, so a centimetre on screen is a real centimetre. If your device isn't listed, calibrate it once with any bank card.
Hold any bank card or driving licence against the outline and slide until the edges line up.