Your pupillary distance (PD) is the distance between your pupils in millimetres. We use it to place the optical centres of the lenses exactly in front of your eyes. An adult PD is typically 54–74 mm.
First, check your glasses card
You can often find the value, marked PD, on the card that came with your previous glasses. If it's there, you can use it as it is. If the card shows only one value, divide it by two (for example PD 58 → 29 for each eye).
Measuring at home
Hold a ruler against your forehead and use a pen to mark on the ruler the centre of both pupils and the centre of the bridge of your nose. For each eye separately, measure the distance from the centre of the pupil to the centre of the bridge of your nose. A small difference between the eyes is normal – it's rarely more than 3 mm.
For progressive glasses it's best to measure the PD separately for each eye, so that the viewing zones of the lenses line up correctly.
You'll get the most exact measurement from an optician.
Detailed instructions and a video guide: banaanieyewear.com/pages/silmateravalimitta