Use a millimeter ruler or PD ruler
Stand about 8 inches from a mirror
Keep your head straight and look at your right eye
Align the 0 mark with the center of your right pupil
Without moving the ruler, look at your left eye
Read the millimeter mark at the center of your left pupil
The number you read is your distance PD
Repeat the measurement 2 to 3 times for accuracy
For single-vision glasses, use one PD number
For bifocal or progressive glasses, measure both distance PD and near PD
Ask an eye care professional to measure your PD if needed