An eye prescription is a set of numbers and abbreviations that describe exactly how your lenses need to be shaped to correct your vision. Once you know what each term 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 eye prescription is split into at least two rows, one for each eye. They’re labeled with Latin abbreviations: OD (oculus dexter) means your right eye, OS (oculus sinister) means your left eye, and OU (oculus uterque) means both eyes. Some newer prescriptions simply use “R” and “L” instead, but the Latin versions are still standard on most forms.
Sphere: Your Main Correction
The sphere column, often shortened to SPH, is the most important number on your prescription. It tells you the overall lens power needed to correct your vision, measured in units called diopters (D). Think of zero as perfect vision sitting in the middle of a number line. The further your number falls from zero in either direction, the stronger your prescription.
A minus sign (like -2.50) means you’re nearsighted, so distant objects look blurry. A plus sign (like +1.75) means you’re farsighted, so close-up objects are harder to focus on. A sphere value between -0.50 and -3.00 is generally considered mild nearsightedness, while anything beyond -6.00 is considered high. The same scale applies on the plus side for farsightedness.
Cylinder and Axis: Astigmatism Correction
If your cornea isn’t perfectly round, you have astigmatism, and two extra numbers will appear on your prescription. The cylinder (CYL) value measures how much astigmatism you have. The American Academy of Ophthalmology describes it this way: the more your eye is shaped like a football instead of a basketball, the higher your cylinder number. Like sphere, cylinder is measured in diopters and can be plus or minus.
The axis number, ranging from 0 to 180 degrees, tells the lab where on your cornea the irregular curve sits. Picture your eye as a clock face: 90 degrees runs straight up and down, while 180 degrees runs side to side. The axis pinpoints the angle so the lens correction lines up precisely with the shape of your cornea. If your cylinder value is zero or blank, you don’t have astigmatism, and there won’t be an axis number.
ADD: The Reading Power
If you’re over 40, your prescription may include a value labeled ADD. This is the extra magnifying power added to the bottom portion of bifocal or progressive lenses to help with close-up tasks like reading. The ADD value is always a plus number (typically between +0.75 and +3.00) and is usually the same for both eyes. It compensates for presbyopia, the gradual loss of your eye’s ability to shift focus to nearby objects as you age.
Prism: Correcting Double Vision
Most prescriptions don’t include prism, but if yours does, it means your eyes aren’t perfectly aligned and you’re seeing double. A prism built into the lens bends light before it enters your eye, redirecting it to the correct spot on your retina so your brain can merge the images from both eyes into a single, clear picture. Prism strength is measured in prism diopters (written as 0.5PD, 1.0PD, and so on), and the prescription will also note the direction the prism base should face: up, down, in, or out.
Pupillary Distance
Pupillary distance (PD) is the measurement, in millimeters, between the centers of your two pupils. It determines where the optical center of each lens should sit. When the lens center lines up precisely with your pupil, you get the clearest, most comfortable vision. Even a small error can introduce distortion, eye strain, and headaches.
PD is sometimes listed on your prescription, but many eye doctors don’t include it automatically. If you’re ordering glasses online, you’ll need this number. The average adult PD falls between about 54 and 74 mm. Your optometrist can measure it for you, or some online retailers provide tools to measure it at home.
Visual Acuity: What 20/20 Actually Means
Your prescription corrects your vision toward a target, and that target is expressed as visual acuity. The familiar fraction works like this: the top number is the distance you stand from the eye chart (usually 20 feet), and the bottom number is the distance at which someone with normal eyesight could read that same line. So 20/20 means you see at 20 feet what a person with normal vision sees at 20 feet. If your acuity is 20/40, you need to be at 20 feet to read what someone with normal vision reads from 40 feet away.
For reference, statutory blindness is defined as a best-corrected visual acuity of 20/200 or worse in the better eye, or a visual field narrowed to 20 degrees or less. Your prescription’s goal is to bring your corrected acuity as close to 20/20 as possible.
How Long Your Prescription Lasts
Eye prescriptions expire. Most states set expiration dates of one to two years for glasses and one year for contact lenses. The expiration date should be printed on the prescription itself. After that date, you’ll need a new eye exam before ordering replacement lenses. Your vision can change gradually, so an expired prescription may no longer give you the sharpest correction.
Glasses vs. Contact Lens Prescriptions
A glasses prescription and a contact lens prescription are not interchangeable. Contact lenses sit directly on your cornea, while glasses sit about 12 mm in front of your eye. That distance changes the effective power of the lens, so the numbers differ, especially for stronger prescriptions. A contact lens prescription also includes a base curve (the curvature of the lens) and a diameter to fit your eye properly. You need a separate fitting and prescription for contacts even if you already have a glasses prescription.
Protecting Your Eyes Between Exams
If you spend long hours reading or looking at screens, your eyes work harder to maintain focus at close range. The 20-20-20 rule, recommended by the Mayo Clinic, is a simple way to reduce strain: every 20 minutes, look at something 20 feet away for at least 20 seconds. This gives the focusing muscles inside your eye a brief rest and can help prevent the headaches and blurry vision that come with prolonged near work.