Stand about 8–10 inches from a mirror
Hold a ruler against your brow line
Close your right eye and align the 0 mm mark with the center of your left pupil
Close your left eye and open your right eye
Read the millimeter mark that lines up with the center of your right pupil
Use that measurement as your pupillary distance
Ask someone else to measure from the center of one pupil to the center of the other pupil
Use a pupillary distance app or online tool with a camera
Check your eyeglass prescription, if it includes pupillary distance
Measure multiple times and use the average if needed