Stand in front of a mirror
Hold a ruler across the bridge of your nose
Close your right eye
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
The number in millimeters is your PD
Repeat the measurement a few times for accuracy
Have someone else measure if possible
Use the PD listed on a recent eyeglass prescription if available
Ask an optician or eye doctor to measure it if needed