To order glasses online, you need three things: a valid prescription, your pupillary distance (PD), and your frame size measurements. Your prescription covers the lens power, but the measurements ensure the glasses actually fit your face and that the lenses are centered correctly over your eyes. Missing or inaccurate measurements are the most common reason online glasses don’t feel right.
Pupillary Distance (PD)
Pupillary distance is the single most important measurement for online glasses orders. It’s the distance in millimeters between the centers of your pupils, and it tells the lab exactly where to place the optical center of each lens. When the optical center lines up with your pupil, you get the sharpest, most comfortable vision. When it’s off, you may experience eye strain, headaches, or blurry spots.
The average adult PD falls between 54 and 74 mm. You’ll encounter two types:
- Single (binocular) PD: One number representing the total distance from pupil to pupil, like 63 mm.
- Dual (monocular) PD: Two numbers measuring the distance from each pupil to the center of your nose bridge, written as something like 31/30. The first number is your right eye, the second is your left. Dual PD is more precise because most people’s faces aren’t perfectly symmetrical.
Your eye doctor may or may not include PD on your prescription, since it’s technically a fitting measurement rather than a prescription value. If it’s not listed, you can ask the office to add it, or measure it yourself at home using a millimeter ruler and a mirror. Many online retailers also offer apps or virtual tools that calculate PD from a photo of your face.
If you’re ordering progressive (multifocal) lenses, a dual PD measurement is the better choice. It gives the lab finer control over where the reading zone sits in each lens.
Frame Size: The Three Numbers on Your Glasses
Look on the inside of either temple arm of a pair of glasses that fits you well. You’ll find a series of numbers, usually separated by dashes or a small square symbol. These three numbers, always in the same order, are your frame size blueprint.
- Lens width (eye size): The first number, typically 40 to 60 mm. This is the horizontal width of each lens opening, not including the frame material around it. It’s the biggest factor in how wide the glasses look on your face.
- Bridge width: The second number, typically 14 to 24 mm. This is the distance across the nose piece between the two lenses. A smaller bridge suits a narrower nose; a larger one fits a wider nose. Getting this wrong is a common reason glasses slide down or pinch.
- Temple length: The third number, typically 120 to 150 mm. This measures the full length of each arm, from the hinge at the front all the way to the curved tip behind your ear. Most temples come in standard increments (135, 140, 145, 150), so you usually don’t need to be exact to the millimeter.
If you don’t own a pair of glasses you like the fit of, many online retailers publish the lens width, bridge width, and temple length for every frame they sell. You can compare those numbers to a pair you’ve tried on at a store, or use printable sizing guides to get in the right range. Getting within 1 to 2 mm on lens width and bridge width will generally give you a comfortable fit.
Extra Measurements for Progressive or Bifocal Lenses
If you’re ordering single-vision lenses, the measurements above are all you need. Progressive and bifocal lenses require one more: segment height.
Segment height is the vertical distance from the bottom edge of the lens opening up to the center of your pupil while you’re wearing the frame. It tells the lab where to position the reading zone so you can look down naturally and hit the right part of the lens. This measurement changes depending on the frame, so you need to determine it with the specific frame you’re ordering, not just any pair you own.
To measure it at home, you’ll need the frame (or a frame with the same dimensions), a non-permanent marker, and a mirror. Look straight ahead, mark a dot on each lens at your pupil center, then measure from the lowest inside edge of the lens opening straight up to the dot. Some online retailers handle this step for you by calculating segment height from your PD and the frame’s dimensions, but others require you to enter it manually.
Frame height matters here too. Progressive lenses need enough vertical space to fit the distance, intermediate, and reading zones without cramming them together. Most standard progressive designs need a frame with at least 28 to 30 mm of vertical lens height. Some compact designs work in frames as shallow as 18 mm, but the reading area will be smaller and the transitions between zones tighter. If you prefer narrow, rectangular frames, check whether the retailer offers short-corridor progressive lenses before ordering.
Your Prescription Details
Your prescription itself isn’t a measurement you take, but you do need to enter it correctly. Every online retailer will ask for these values from your prescription form:
- Sphere (SPH): The main correction for nearsightedness (negative number) or farsightedness (positive number).
- Cylinder (CYL) and Axis: These two always go together and correct astigmatism. If your prescription has no cylinder value, you don’t have astigmatism and can skip both.
- Add power: Only present if you need multifocal lenses. It’s the extra magnification for reading, written as a positive number like +2.00.
Prescriptions expire after one to two years depending on your state, and online retailers will ask for the date. An expired prescription won’t be accepted. Double-check that you’re entering the right values for the right eye (OD) and left eye (OS), since swapping them is an easy mistake that results in completely wrong lenses.
Quick Reference Checklist
For single-vision glasses, gather these before you start your order:
- Valid prescription with SPH, CYL, Axis (if applicable)
- Pupillary distance (single or dual)
- Lens width from a well-fitting pair
- Bridge width from a well-fitting pair
- Temple length from a well-fitting pair
For progressive or bifocal lenses, add:
- Add power from your prescription
- Segment height measured with your chosen frame
- Dual PD (preferred over single PD for multifocals)
Having all of these ready before you start shopping saves time and reduces the chance of ordering errors. Write them down or photograph your current frames’ temple markings and your prescription so everything is in one place when you need it.