Size is one of the most important physical features when identifying and characterizing an organism in the lab. The exact size of an organism can be determined using a calibrated ocular micrometer. An ocular micrometer is a calibrated ruler etched on a glass disk that is used for measuring organisms under a microscope. The ocular micrometer sits inside the eyepiece. This video provides step-by-step instructions on how to calibrate an ocular micrometer on a compound microscope with mathematical calculations. It also shows how to use the stage micrometer, which is a special glass slide clipped to the stage of the slide, to calibrate the ocular micrometer. Calibration should be done separately for each objective lens and different microscopes.