Stand about 8 inches from a mirror
Hold a ruler against your brow line, centered over the bridge of your nose
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
Record that number as your pupillary distance
Repeat the measurement 2 to 3 times for accuracy
Ask another person to measure from the center of one pupil to the center of the other if needed
Use a smartphone PD measurement app if you do not have a mirror and ruler setup
Measure in good lighting and keep your head straight
For single-vision glasses, use the full distance between pupils
For bifocal or progressive lenses, ask for both distance PD and near PD if needed