When you order for glasses online, you are generally asked for PD which means your "Pupillary Distance". It is very essential to know the measurement before you book your spectacles. It is quite easy to measure the distance, there is not even the need to visit the optician, you can do it at your home itself.
You will just need a ruler and a friend to take the measurement.
Another easy way to do it is by standing in front of mirror. Wear any existing pair of glasses and stand 3 to 4 feet away from the mirror. Lightly close your left eye with some thick paper. Take a marker and mark the centre part of your right pupil. Repeat this with your left eye and finally measure the distance; you will get your PD.
Mostly the PD of adults is between 55 and 65 while of children it is between 42 and 54.