Tip: hold a real ruler or credit card (85.6mm wide) to the screen and adjust until the marks line up.
Measure small objects right against your screen with an on screen ruler marked in both centimeters and inches. Because every display has a different pixel density, the ruler includes a calibration slider: hold a real credit card or a physical ruler against the screen and adjust the slider until the markings match, and from then on your measurements will be accurate. This is handy for a quick measurement when you cannot find a tape measure, for checking a length while shopping online, or for design work. Keep in mind that accuracy depends entirely on calibrating for your specific monitor, and that very large objects will not fit. Everything runs locally in your browser, and your calibration setting is remembered so you only have to set it once.