Stand about 8 to 10 inches away from a mirror
Hold a ruler against your brow line
Close your right eye and align the zero mark with the center of your left pupil
Open your right eye and close your left eye
Read the measurement at the center of your right pupil
Repeat the process a few times for accuracy
Ask someone 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 you do not have a ruler and mirror
Have an optician measure your pupillary distance during an eye exam if you want the most accurate result