A glasses prescription is a small grid of abbreviations and numbers that tells a lens maker exactly how to correct your vision. Once you know what each abbreviation stands for and what the numbers mean, the whole thing takes about 30 seconds to read.
OD and OS: Which Eye Is Which
The first thing you’ll notice on your prescription are the labels OD and OS. OD stands for “oculus dexter,” the Latin term for your right eye. OS stands for “oculus sinister,” your left eye. Some prescriptions also include OU, short for “oculus uterque,” meaning both eyes. The right eye (OD) is almost always listed on the top row, and the left eye (OS) on the bottom.
Newer prescriptions sometimes skip the Latin and simply label the rows “Right” and “Left,” but the layout is the same either way.
Sphere (SPH): Your Main Correction
The sphere column is the core of your prescription. It measures how much corrective power your lenses need, expressed in diopters. A negative number means you’re nearsighted (you see close objects clearly but distant ones are blurry). A positive number means you’re farsighted (distant objects may be clearer than close ones).
The further the number is from zero, the stronger your prescription. For nearsightedness, the ranges break down like this:
- Mild: -0.25 to -3.00 diopters
- Moderate: -3.25 to -6.00 diopters
- High: beyond -6.00 diopters
For farsightedness:
- Mild: +0.25 to +2.00 diopters
- Moderate: +2.25 to +4.00 diopters
- High: above +4.00 diopters
Prescriptions change in increments of 0.25 diopters. So you might see -1.75 or +2.50, but never -1.30. If the sphere column says “Plano” or “PL,” it means that eye needs no spherical correction, though it may still need correction for astigmatism.
Cylinder (CYL) and Axis: Astigmatism
If your eye isn’t perfectly round, light bends unevenly as it enters, creating blurry or distorted vision at certain angles. This is astigmatism, and it’s extremely common. Two numbers on your prescription work together to correct it.
The cylinder value measures how much astigmatism correction you need, also in diopters. It can be written as a negative or positive number depending on the convention your eye doctor uses. A cylinder of -0.50 is very mild; -2.00 or beyond is more significant.
The axis is a number between 1 and 180 that indicates the angle, in degrees, at which the cylindrical correction should be placed in the lens. Think of it as telling the lens maker which direction the unevenness runs. The axis has no relationship to how strong your prescription is. It’s purely directional. If your cylinder column is blank or marked “DS” (diopters sphere), you have little to no astigmatism and can ignore the axis entirely.
ADD Power: The Reading Portion
If you’re over 40, your prescription may include an “ADD” value. This is the additional magnifying power added to the bottom portion of bifocal or progressive lenses so you can see things up close, like a phone screen or a book. The ADD number is always positive and typically falls between +0.75 and +3.00 diopters. It’s usually the same for both eyes.
The ADD value reflects a normal age-related change in the lens inside your eye, which gradually loses flexibility and makes it harder to focus on nearby objects. A higher ADD number simply means your eyes need more help with close-up tasks.
Prism: Correcting Eye Alignment
Most prescriptions don’t include a prism value, but if yours does, it means your eyes don’t align perfectly with each other, which can cause double vision or eye strain. A prism lens bends light before it reaches your eye so both eyes receive the image in the same spot.
Prism is measured in prism diopters and paired with a base direction that tells the lab which way to orient the thicker edge of the prism. The four base directions are Base Up (BU), Base Down (BD), Base In (BI, toward your nose), and Base Out (BO, toward your ear). You might see something like “1.5 BU” on one eye and “1.5 BD” on the other, working together to bring your vision into alignment.
Pupillary Distance (PD)
Pupillary distance is the measurement, in millimeters, between the centers of your two pupils. It tells the lab exactly where to place the optical center of each lens so the corrective power lines up with where you actually look through the glasses. Without an accurate PD, lenses can cause headaches, eye strain, and blurry vision even if every other number is correct.
For adults, PD generally falls between 46 and 74 millimeters. Women typically measure between 46 and 65 mm, and men between 59 and 74 mm. Your PD may be written as a single number (the distance between both pupils) or as two separate numbers (one for each eye measured from the center of the nose). Some doctors include PD on the prescription itself; others record it separately, and you may need to ask for it if you plan to order glasses online.
A Sample Prescription, Decoded
Here’s what a typical prescription might look like and what each piece means:
OD: -2.50 -0.75 x 180
OS: -3.00 -0.50 x 170
ADD: +2.00
PD: 63
The right eye is nearsighted at -2.50 diopters with mild astigmatism (-0.75 cylinder) oriented at 180 degrees. The left eye is slightly more nearsighted at -3.00, with a smaller astigmatism correction (-0.50) at 170 degrees. Both eyes get an additional +2.00 for reading, and the lenses should be centered for a pupillary distance of 63 mm.
Glasses Prescriptions vs. Contact Lens Prescriptions
A glasses prescription and a contact lens prescription are not interchangeable, even if your eye doctor wrote both on the same visit. Because contact lenses sit directly on your eye rather than about 12 millimeters in front of it, the corrective power often differs slightly. Contact lens prescriptions also include measurements that glasses prescriptions don’t: a base curve (BC), which describes the curvature of the lens to match your eye’s shape, and a diameter (DIA), the overall width of the lens. You cannot use a glasses prescription to order contacts, or vice versa.
Your Right to Your Prescription
Under the FTC’s Eyeglass Rule, optometrists and ophthalmologists must give you a copy of your prescription at the end of your eye exam at no extra charge. They also cannot require you to buy glasses from them as a condition of getting the exam. The prescription is yours, and you can fill it wherever you choose, whether that’s your doctor’s office, a retail chain, or an online retailer.
Glasses prescriptions typically expire after one to two years, depending on your state’s regulations. After that, you’ll need a new eye exam to get an updated prescription. If your vision changes noticeably before the expiration date, it’s worth getting checked sooner rather than waiting.