1. Stand about 20 cm away from a mirror with your face parallel to the mirror. Hold the ruler below your eyes, keeping it exactly horizontal.
2. Align the ruler’s zero (0) measurement to the center of your left pupil, then check the position of the center of the pupil of the left eye and zero (0) measurement with both eyes. After that, you can look at the right eye with both eyes, and read the millimeter line under the center of the pupil of the right eye. This is your PD measurement.
3. Repeat these steps at least 3 times to get an accurate and consistent measurement.