An eye prescription is a small grid of numbers and abbreviations that tells a lab exactly how to grind your lenses. Once you know what each column means, the whole thing takes about 30 seconds to read. Here’s a breakdown of every value you’ll find on a typical prescription and what it tells you about your vision.
OD, OS, and OU: Which Eye Is Which
Every prescription starts with at least two rows, one for each eye. OD stands for the Latin “oculus dexter,” meaning right eye. OS stands for “oculus sinister,” meaning left eye. If you see OU, that means “both eyes” and indicates a value that applies to both. Your right eye (OD) is almost always listed first.
Sphere (SPH): Your Main Correction
The sphere column is the most important number on your prescription. It measures how much correction your eyes need, expressed in units called diopters. The sign in front of the number tells you the type of vision problem being corrected:
- Minus sign (−) means you’re nearsighted. You see well up close but need help with distance. A prescription of −2.50 is a moderate correction; −9.00 is a strong one.
- Plus sign (+) means you’re farsighted. You see better at a distance but need help with close-up tasks.
The higher the number (ignoring the sign), the stronger your prescription. A sphere of −1.00 is mild. A sphere of −6.00 or higher is considered a strong prescription. If this box says “PL” or “Plano,” it means zero correction is needed for that measurement, and your eye is essentially normal in that regard.
Cylinder (CYL) and Axis: Astigmatism Correction
These two columns always work as a pair. If they’re blank, you don’t have astigmatism and can skip this section.
The cylinder value measures how much astigmatism you have. Astigmatism means your cornea (the clear front surface of your eye) isn’t perfectly round. Instead, it’s slightly oblong, like a football, which causes light to focus unevenly. The CYL number, also measured in diopters, corrects for that second, different curve.
The axis tells the lab where the irregular curve sits on your cornea. It’s measured in degrees from 1 to 180, like the bottom half of a protractor laid across your eye. An axis of 90 means the astigmatism runs vertically. An axis of 180 means it runs horizontally. This number has to be exact, because even a few degrees off can make your vision blurry.
If your prescription reads, for example, OD −3.00 −1.25 × 170, that means your right eye has 3 diopters of nearsightedness, 1.25 diopters of astigmatism, and the astigmatism is oriented at 170 degrees.
ADD Power: The Reading Boost
If you’re over 40, there’s a good chance your prescription includes an ADD column. This is extra magnifying power added to the bottom portion of your lenses (in bifocals or progressives) so you can read up close. It compensates for presbyopia, the gradual stiffening of the lens inside your eye that makes it harder to focus on nearby objects as you age.
ADD values are always positive and typically range from +0.50 to +3.00. A lower number like +1.00 means you need just a slight boost for reading. A higher number like +2.50 means your eyes need significantly more help with close-up tasks. The ADD value is usually the same for both eyes.
Prism and Base: Alignment Correction
Most prescriptions leave this section blank. Prism correction is only needed when your eyes don’t aim at the same point, causing you to see double. The prism bends light before it enters your eye so that both eyes receive the image in the right spot, and your brain can merge them into a single, clear picture.
Prism strength is measured in prism diopters (sometimes written as a triangle symbol, Δ). Next to the prism value, you’ll see a base direction that tells the lab which edge of the lens should be thickest:
- BU (Base Up) shifts the image downward.
- BD (Base Down) shifts the image upward.
- BI (Base In) shifts the image toward the nose.
- BO (Base Out) shifts the image toward the ear.
Pupillary Distance (PD)
Pupillary distance is the space between the centers of your two pupils, measured in millimeters. It ensures the optical center of each lens lines up with your eye. If the PD is off, you may experience eyestrain or blurry vision even with the correct prescription.
You’ll need your PD to order glasses online, but it’s not always printed on your prescription. Some offices measure it only when you purchase glasses through them, and getting it separately may cost a small fee. You can measure it yourself with a millimeter ruler and a mirror: close one eye, align the ruler’s zero mark with the center of one pupil, then open the other eye and read the millimeter mark at the center of the other pupil. Most adults have a PD somewhere between 54 and 74 mm.
NV and DV: Distance vs. Near
Some prescriptions split your correction into two sections labeled DV (distance vision) and NV (near vision) instead of using a single grid with an ADD column. The DV section contains your sphere, cylinder, and axis for seeing things far away. The NV section shows the adjusted values for reading and close-up work. These are just two ways of expressing the same information. If your prescription uses the ADD format, the near-vision power is your sphere value plus the ADD number.
Contact Lens Prescriptions Are Different
You cannot use a glasses prescription to buy contact lenses. Because contacts sit directly on your eye rather than a short distance in front of it, the power values are often different. A contact lens prescription also includes two measurements that glasses prescriptions don’t have:
- Base Curve (BC) describes how curved the back surface of the lens is, so the contact fits the shape of your cornea.
- Diameter (DIA) is the width of the lens from edge to edge.
Contact lens prescriptions also specify a brand or material, because different lenses fit differently even with the same numbers. Your eye doctor determines these values during a separate contact lens fitting.
Your Right to Your Prescription
Under the FTC’s Eyeglass Rule, your eye doctor is required to hand you a copy of your glasses prescription immediately after your exam, free of charge, whether you ask for it or not. They cannot require you to buy glasses from them first, charge you extra for the prescription itself, or make you sign a waiver as a condition of receiving it. This rule exists so you can shop for glasses wherever you choose, including online retailers. If your doctor’s office doesn’t automatically hand you your prescription, they’re not following federal law.