Your eye exam results are a grid of numbers and abbreviations that describe exactly how your eyes focus light and how healthy they are. Once you know what each value means, the whole prescription becomes straightforward to read. Here’s how to decode every part of it.
OD, OS, and OU: Which Eye Is Which
Every eye prescription starts by separating your two eyes. OD stands for “oculus dexter,” your right eye. OS stands for “oculus sinister,” your left eye. If you see OU, that means both eyes together. These abbreviations appear as row labels on your prescription, with each row containing the specific correction values for that eye.
Sphere (SPH): Your Basic Lens Power
The sphere value is the main number on your prescription, measured in diopters. It tells you how much correction you need to see clearly at distance. A minus sign means you’re nearsighted (you see close objects well but distant ones are blurry). A plus sign means you’re farsighted (distant objects may be clearer than close ones).
The farther the number is from zero, the stronger your prescription. Someone with -1.00 has mild nearsightedness, while -6.00 is a significantly stronger correction. If the cylinder column on your prescription says “DS” (for “diopter sphere”), it simply means you have no astigmatism in that eye and only need the sphere correction.
Cylinder (CYL) and Axis: Astigmatism Correction
If your cornea is shaped more like a football than a basketball, light bends unevenly and causes blurry or distorted vision at all distances. That’s astigmatism, and it gets its own pair of values on your prescription.
The cylinder number indicates how much astigmatism correction you need, also measured in diopters. It can be written as a minus or plus value depending on the notation style your doctor uses. The axis is a number between 1 and 180 that describes the angle, in degrees, where the astigmatism correction should be oriented on your lens. Think of it as telling the lab which direction to position the correction. A cylinder of -0.75 at axis 90, for example, means a small amount of astigmatism corrected along the vertical meridian of the lens. If both the cylinder and axis columns are blank, you don’t have astigmatism.
Add Power: The Reading Correction
If you’re over 40, your prescription may include a value labeled “Add.” This is additional magnifying power built into the lower portion of your lenses for reading and close-up work. It compensates for presbyopia, the gradual loss of your eye’s ability to focus on nearby objects that happens naturally with age.
The Add value is always a plus number, typically ranging from +0.75 to +3.00. A higher number means you need more help focusing up close. This value is the same for both eyes in most prescriptions, and it’s what makes bifocals, progressives, and reading glasses work. If your prescription doesn’t have an Add column, you don’t need a separate reading correction.
Prism and Base: Correcting Double Vision
Most prescriptions don’t include prism, but if yours does, it means your eyes aren’t aligning properly and you’re seeing double images. A prism built into the lens redirects light so both eyes receive the image in the same place, merging double vision into a single clear picture.
The prism value tells you how much light redirection is needed. The base tells you which direction. You’ll see one of four abbreviations:
- BU (Base Up): the thickest part of the prism is at the top of the lens
- BD (Base Down): the thickest part is at the bottom
- BI (Base In): the thickest part faces your nose
- BO (Base Out): the thickest part faces your ear
Visual Acuity: The 20/20 Number
Your visual acuity score comes from the letter chart you read during the exam. It’s written as a fraction like 20/20, 20/40, or 20/15. The first number is always 20 because the test places you 20 feet from the chart. The second number tells you how your sharpness compares to normal vision.
With 20/20 vision, you see at 20 feet what a person with normal clarity sees at 20 feet. If you have 20/40 vision, what you can read at 20 feet is what someone with normal vision could read from 40 feet away, meaning your vision is worse than average. A score of 20/15 is actually better than normal: you see at 20 feet what most people would need to be 15 feet away to see.
This number matters practically. Nearly every U.S. state requires best-corrected visual acuity of 20/40 or better in your stronger eye to hold an unrestricted driver’s license. Your doctor may note your acuity both with and without correction so you can see how much your glasses or contacts improve things.
Eye Pressure (IOP)
During a comprehensive eye exam, your doctor measures the pressure inside your eyes, called intraocular pressure. Normal eye pressure falls between 10 and 20 mmHg. This measurement is a screening tool for glaucoma, a condition where elevated pressure can damage the optic nerve over time. A single reading above 20 doesn’t automatically mean you have glaucoma, but it usually triggers additional testing. Your results sheet may list this as “IOP” or “Tonometry.”
Pupillary Distance (PD)
Pupillary distance is the space between the centers of your two pupils, measured in millimeters. The average adult PD is about 63 mm, though it can range from around 50 mm to 70 mm. This number doesn’t describe your vision, but it’s essential for making your glasses. It tells the lab exactly where to position the optical center of each lens so the correction lines up precisely with your pupils. If the alignment is off, you can experience eye strain, headaches, or blurry spots even with the correct prescription.
Some doctors include PD on your prescription automatically. Others don’t, and you may need to ask for it, especially if you’re ordering glasses online.
Why Glasses and Contact Prescriptions Differ
If you wear both glasses and contacts, you’ve probably noticed the numbers aren’t the same. That’s because glasses sit about 10 to 14 millimeters in front of your eyes, while contacts rest directly on the cornea. This gap, called vertex distance, changes how much power the lens effectively delivers to your eye. The farther a lens sits from the cornea, the more plus power it produces.
For mild prescriptions (under about 4.00 diopters in either direction), the difference is negligible. Once the prescription is stronger than that, a contact lens prescription needs to be mathematically adjusted to account for the shorter distance. This is why you can’t simply use your glasses prescription to order contacts. A contact lens fitting also includes measurements specific to your cornea’s shape and diameter that don’t appear on a glasses prescription at all.
Putting It All Together
Here’s what a typical glasses prescription looks like when you read it left to right:
OD: -2.50 -0.75 x 180
OS: -3.00 -0.50 x 170
Add: +2.00
PD: 64
This person is nearsighted in both eyes (minus sphere values), with mild astigmatism in each eye (the cylinder and axis values). They need a reading addition of +2.00, suggesting they’re over 40 with some presbyopia. Their pupillary distance is 64 mm. The right eye has a slightly weaker prescription than the left, which is completely normal. Most people have some difference between their two eyes.
Your prescription typically expires after one to two years, depending on your state’s regulations and your doctor’s recommendation. The expiration date is printed on the prescription itself. After that, you’ll need a new exam before ordering updated lenses.