The Measurements We Need: PD and Segment HeightUpdated 2 hours ago
Along with your prescription, we need one or two measurements from you. Your prescription says what power your lenses need. These measurements tell us where that power has to sit in your frame.
What Is Pupillary Distance (PD)?
Your pupillary distance is the distance between the centers of your pupils, measured in millimeters. We use it to line up the optical center of each lens with each eye.
This matters more than it sounds like it should. If the optical centers sit off to one side of where your eyes actually are, you end up looking through the wrong part of the lens, which can cause eye strain, headaches or a pulling sensation even though the prescription itself is correct.
A PD is required on every prescription order.
Single PD or Dual PD?
We accept both.
A single PD is one number, such as 63, measuring pupil to pupil. It is the most common way a PD is written.
A dual PD is two numbers, such as 32.0 / 31.5, measuring each pupil from the center of your nose. It is more precise, and some prescriptions are written this way.
Send whichever one you have. If your prescription lists a dual PD, use it rather than adding the halves together.
How Do I Find My PD?
Check your prescription first. If a PD is printed on it, use that number.
Ask your eye doctor's office. They measured it during your exam, and most will provide it on request.
Measure it at home. You can take your own PD with a millimeter ruler and a mirror. We have a printable ruler guide that walks through it. The short version:
Stand about 8 inches (20 cm) from a mirror, or from a friend.
Hold the ruler just below your eyes, facing straight ahead.
Close your left eye and line the ruler's zero up with the center of your right pupil.
Still looking straight ahead, close your right eye and open your left.
Read the millimeter line sitting directly over the center of your left pupil. That is your PD.
Repeat three or four times to make sure you get a consistent number.
An adult PD usually falls between 58 and 65. Having a friend measure for you is more accurate than doing it in a mirror.
What Is Segment Height?
Segment height is how far up the lens your reading zone begins, measured from the bottom of the lens to the center of your pupil. It is only needed for progressive lenses.
Because a progressive lens changes power from top to bottom, this measurement decides where your distance vision ends and your reading vision begins in your specific frame. Two people with identical prescriptions can need different segment heights simply because their frames sit differently on their faces.
Segment height is usually not printed on a prescription, because it depends on the frame rather than on your eyes.
How Do I Measure My Segment Height?
You will need the frames you are sending us, a millimeter ruler and a mirror. We have a printable guide for this as well. The short version:
Put your glasses on and stand about 8 inches (20 cm) from a mirror, or from a friend.
Facing straight on, hold the ruler vertically between your nose and your pupil.
Line the ruler's 0 mm mark up with the bottom of the lens, not the bottom of the frame.
Still looking straight ahead, read the millimeter mark that lines up with the center of your pupil. That is your segment height.
Repeat three or four times to make sure you get a consistent number.
Measure while wearing the frames the way you normally wear them. Pushing them up or holding them differently than usual will shift the result.
Why Accuracy Matters
Both of these measurements are entered by you at checkout, and we build your lenses to them. A measurement that is off by a few millimeters can put the wrong part of the lens in front of your eye.
Take each measurement a few times, and if the numbers do not agree, take them again. If you are not confident in a measurement, our Rx Eyewear Experts would much rather help you before your order goes into production than after.