Stand about 8 inches from a mirror
Hold a millimeter ruler against your brow
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 at the center of your right pupil
The number you read is your PD in millimeters
For distance PD, look at a distant object while measuring
For near PD, look at something about 40 cm away while measuring
Repeat the measurement 2 to 3 times and use the average
Ask someone else to measure if you want a second check
Use an online PD tool or app if you do not have a ruler
Have an optician measure your PD for the most accurate result