An optical prescription is a small grid of numbers and abbreviations that tells a lens maker exactly how to correct your vision. Once you know what each column means, the whole thing takes about 30 seconds to decode. Here’s how to read every part of it.
OD, OS, and OU: Which Eye Is Which
Every prescription has at least two rows of numbers, one for each eye. They’re labeled with Latin abbreviations: OD (oculus dexter) is your right eye, and OS (oculus sinister) is your left eye. If you see OU (oculus uterque), that applies to both eyes. On some newer prescriptions, the doctor simply writes “Right” and “Left,” but OD and OS are still the standard on most forms.
Sphere (SPH): Your Main Correction
The sphere column is the core of your prescription. It tells the lens maker how much light-bending power your lenses need, measured in units called diopters (D). The number will have either a minus or plus sign in front of it, and that sign matters a lot.
A minus sign (−) means you’re nearsighted. You see things clearly up close but need correction for distance. A plus sign (+) means you’re farsighted, seeing better at a distance but struggling with things nearby. The higher the number, the stronger your correction. Someone with −1.00 has a mild prescription; someone with −6.00 has a strong one.
If you see “PL” or “Plano” in the sphere column, that eye doesn’t need spherical correction. It’s essentially zero.
Cylinder (CYL) and Axis: Astigmatism Correction
These two columns always work as a pair. If you have astigmatism, your eye’s surface is shaped more like a football than a basketball, which bends light unevenly. The cylinder value measures how much extra correction is needed to compensate, and the axis (a number between 1 and 180) tells the lab which angle to orient that correction.
Cylinder values also carry a plus or minus sign and are measured in diopters. If both columns are blank, you don’t have astigmatism. The axis number isn’t a strength measurement. It’s a degree of rotation, like a position on a clock face, so the lab knows exactly where on the lens to place the correction.
ADD: The Reading Power
The ADD column appears on prescriptions for people who need help with close-up vision, typically adults over 40 whose eyes have lost some ability to shift focus between distances. This is called presbyopia, and it’s a normal part of aging.
The ADD value is always a plus number (even if the plus sign isn’t written) and represents the extra magnifying power added to the lower portion of bifocal, trifocal, or progressive lenses. Common values range from +0.75 for early presbyopia to +2.50 or higher for more advanced cases. The ADD number is usually the same for both eyes.
Prism and Base: Eye Alignment
Most prescriptions don’t include prism, but if yours does, it corrects for eye alignment problems where your eyes don’t point at the same spot naturally. Prism is measured in prism diopters and shifts the image in a specific direction so your brain can merge what both eyes see into one picture.
The base tells the lab which direction to orient the prism: BU (base up), BD (base down), BI (base in, toward your nose), or BO (base out, toward your ear). If you don’t see a prism value on your prescription, your eyes align well enough without one.
Pupillary Distance (PD)
Your pupillary distance is the space between the centers of your pupils, measured in millimeters. The average adult PD is about 63 mm, though it can range anywhere from 50 to 70 mm for most people. This number ensures the optical center of each lens lines up directly with your pupil. If it’s off, you can experience eyestrain, headaches, and distorted vision. PD is especially critical for progressive and bifocal lenses, where each zone of the lens needs precise alignment.
Some doctors include PD on your prescription, and some don’t. If it’s not listed, you can ask your eye care provider to measure it or use an online tool, though in-office measurements are more reliable. Your PD changes during childhood but stays stable once you’re an adult.
Why Glasses and Contact Lens Prescriptions Differ
You can’t use a glasses prescription to order contacts, and vice versa. The reason is simple: glasses sit roughly 12 millimeters away from your eye, while contacts rest directly on the surface. That small gap changes how much the lens needs to bend light, so the power values are recalculated using a formula that accounts for that distance. Your contact lens power may be slightly higher or lower than your glasses power as a result.
Contact lens prescriptions also include two measurements that glasses prescriptions don’t. The base curve (BC) describes the curvature of the lens, matched to the shape of your cornea so the contact fits properly. The diameter (DIA) specifies the overall width of the lens. Both are determined during a contact lens fitting and can’t be guessed from your glasses numbers.
What the Numbers Don’t Tell You
Your prescription doesn’t translate directly into a visual acuity score like 20/20. Diopters measure how much correction you need, while 20/20 measures how sharp your vision is with or without that correction. Two people with the same prescription can have different visual acuity depending on factors like the health of their retina and how their brain processes visual information. Rough conversion charts exist, but they’re estimates and aren’t reliable enough for clinical use.
Your prescription also has an expiration date, which varies by state but is typically one to two years. Federal law requires your eye care provider to give you a copy of your prescription at no extra charge after your exam, and they cannot require you to buy glasses or contacts from them as a condition of the exam. You’re free to shop anywhere with that prescription, whether that’s another optical shop or an online retailer.