Learning how to read an eyeglass prescription comes down to a handful of abbreviations arranged in a consistent pattern: one row per eye, with columns for lens power, astigmatism correction, and any extras your eyes need. Once you know what each column means, the form is just a set of instructions any optician or online retailer can fill.
OD and OS: Which Eye Is Which
OD (Oculus Dexter) is your right eye. OS (Oculus Sinister) is your left. Some prescriptions also use OU (Oculus Uterque), meaning both eyes together. The right eye is almost always listed first, so the top row of numbers belongs to OD.
Sphere (SPH): The Main Correction
The sphere value is the core lens power, measured in diopters. The sign tells you what it’s correcting. A minus number means you’re nearsighted, so close-up is clear and distance is blurry. A plus number means you’re farsighted, so distance is clearer and close work is harder. The farther the number sits from zero in either direction, the stronger the lens.1American Academy of Ophthalmology. How to Read an Eyeglasses Prescription
If one eye needs correction and the other doesn’t, you may see “plano” or “PL” in the sphere column, which means zero power.
Cylinder (CYL) and Axis: Astigmatism
Cylinder and axis always work together, and they only appear if you have astigmatism — an irregular corneal shape that bends light unevenly. The cylinder number is how much correction the irregularity needs. The axis, written as a degree between 1 and 180, tells the lab where on the cornea that irregularity sits so the correction is oriented the right way inside the lens.1American Academy of Ophthalmology. How to Read an Eyeglasses Prescription
Blank CYL and axis columns mean you don’t have astigmatism that needs correcting. You won’t lose anything by leaving those fields empty when you order.
Add Power: Bifocals and Progressives
The Add value shows up when you need bifocal or progressive lenses. It’s the extra magnifying power built into the lower part of the lens for reading and other close work. Add is always positive and is usually the same for both eyes. No Add value on your form means single-vision lenses are all you need.
Prism and Base: Alignment Correction
Most prescriptions skip this section. When it is used, prism correction addresses eye alignment rather than focus, bending light so the images from each eye line up and double vision eases. A prism entry has two parts: a strength in prism diopters, and a base direction that tells the lab which way to point the thickest edge of the prism.
- BU (Base Up): thickest edge at the top, shifting the image downward.
- BD (Base Down): thickest edge at the bottom, shifting the image upward.
- BI (Base In): thickest edge toward the nose, shifting the image outward.
- BO (Base Out): thickest edge toward the ear, shifting the image inward.
If your prescription includes prism, confirm your retailer handles prism orders before you buy. Not every budget online vendor does.
Pupillary Distance (PD)
Pupillary distance is the space in millimeters between the centers of your pupils, and the lab uses it to line up the optical center of each lens with your line of sight. Without it, you can end up with eye strain, headaches, or blur even when the sphere, cylinder, and axis are perfect.
Federal law does not require your eye doctor to put PD on the prescription. Only Kansas, Massachusetts, Alaska, and New Mexico require it. Everywhere else, your doctor can leave it off, and the FTC only encourages — not requires — them to share it when they’ve measured it during the exam.2Federal Trade Commission. Complying with the Eyeglass Rule
If your PD isn’t written down, call the doctor’s office and ask — some will give it over the phone if it was recorded. An optical shop can measure it quickly. You can also measure it yourself. Stand about eight inches from a mirror, hold a millimeter ruler against your brow, close your right eye, line the ruler’s zero up with the center of your left pupil, then open your right eye and close your left to read the measurement at the center of your right pupil. Smartphone apps that estimate PD from a selfie are another option, though their accuracy varies.
Expiration Date
Eyeglass prescriptions don’t last forever. There is no single federal rule; each state sets its own window, with most falling between one and two years from the exam date.2Federal Trade Commission. Complying with the Eyeglass Rule Your doctor can also mark an earlier expiration if your eyes need closer monitoring.
Vision drifts, sometimes slowly enough that you don’t notice until the headaches or night-driving trouble arrive. Routine exams also catch conditions like glaucoma and macular degeneration that develop quietly. A reputable retailer or lab will refuse an order placed on an expired prescription and send you back for a current exam.
What’s Not on an Eyeglass Prescription
An eyeglass prescription is not a contact lens prescription, even when both correct the same vision problem. Contacts sit directly on the cornea; glasses sit about 12 millimeters off the eye, and that gap changes the lens power calculation, so the numbers on the two prescriptions are often slightly different.
Contact lens prescriptions also carry measurements an eyeglass form never includes:
- Base curve (BC), the curvature matched to your cornea. For soft lenses this usually falls between 8.3 and 9.0 millimeters.
- Diameter (DIA), the width of the lens edge to edge, typically 13.8 to 14.5 millimeters for soft lenses.
- A specific brand or product name, because base curve, material, and thickness vary between brands and are not interchangeable.
Getting contacts requires a separate fitting appointment where the doctor checks how a trial lens centers, moves, and feels. Your eyeglass prescription alone can’t be used to order contact lenses.
Putting the Numbers to Work
Before ordering, read each value off your paper or digital form and check it against what you type into the order screen. Transposing a sphere or cylinder number, or entering the wrong axis, produces lenses that are wrong for your eyes even though every other step was correct. Make sure you have your PD on hand, confirm single-vision versus bifocal or progressive based on whether you see an Add value, and flag any prism correction with your retailer up front. The numbers themselves are simple once you’ve read them a few times; the care goes into transcribing them.