Ask your eye doctor or optician for your PD measurement
Check your eyeglass prescription for a listed PD value
Use a millimeter ruler and a mirror
Hold the ruler across the bridge of your nose
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 millimeter mark at the center of your right pupil
Repeat the measurement a few times for accuracy
Have someone else measure from the center of one pupil to the center of the other
Use a PD measurement app or online tool
Measure near PD and far PD if needed for bifocal or progressive lenses