Stand about 8 inches away from a mirror
Hold a millimeter ruler against your brow line
Close your right eye and align the 0 mm mark with the center of your left pupil
Open your right eye and close your left eye
Read the millimeter mark that lines up with the center of your right pupil
Repeat the measurement a few times for accuracy
Ask someone else to measure from the center of one pupil to the center of the other if needed
Use a pupillary distance app or online tool if a ruler is not available
Record the final measurement in millimeters