You can do it yourself or have someone else measure it using a mirror and a ruler. Place a ruler, millimeter side up, directly under the center of your right or left pupil. While looking straight ahead, measure the distance from the center of your left pupil to the center of your right pupil. Repeat for accuracy. Note that the average PD is between 57 and 65mm. https://www.sllac.com/help/how-to-measure-your-pd