You just finished your eye exam. The doctor hands you a slip of paper covered in abbreviations and numbers, says “take care,” and walks out. You stare at it. OD: -2.25 -0.75 x 180. OS: -1.75. PD: 63. It might as well be a binary code.
You’re not alone. Most people leave eye appointments with their prescription in hand and zero idea what it says. Here’s a plain-English breakdown of every number on that slip — and what it means for the lenses going into your glasses or floating on your eyeballs.
OD and OS: Which Eye Is Which?
These two come from Latin, which is unhelpful but traditional.
OD = oculus dexter = your right eye OS = oculus sinister = your left eye
Some prescriptions also use OU (oculus uterque) for values that apply to both eyes — you’ll sometimes see this for the ADD power.
Your right eye almost always appears first on the form. A blank column for one eye means either no correction is needed for that parameter, or it’s zero.
SPH (Sphere): Your Basic Nearsighted or Farsighted Correction
SPH is the main number — the lens power your eye needs to focus correctly. It’s measured in diopters (D), always written with a + or – sign.
- Negative numbers (–0.25, –3.00, –6.50) = nearsightedness (myopia). Your eye bends light too strongly, blurring distant objects.
- Positive numbers (+1.00, +2.50) = farsightedness (hyperopia). Your eye doesn’t bend light quite enough — nearby objects blur first, and distance can blur too at higher corrections.
The further from zero, the stronger the prescription. Someone at –1.00 has mild myopia. Someone at –8.00 has high myopia and can’t read a clock across the room without correction.
| Myopia Level | SPH Range | Practical Impact |
|---|---|---|
| Mild | –0.25 to –3.00 | Clear up close; blurry beyond a few feet |
| Moderate | –3.00 to –6.00 | Clear only at reading distance without correction |
| High | –6.00 to –10.00 | Blurry beyond arm’s length; higher complication risk |
| Very High / Extreme | Beyond –10.00 | Requires specialty lens materials; increased retinal risks |
| Mild Farsighted | +0.25 to +2.00 | Often no correction needed in youth; eye strain with age |
| Moderate Farsighted | +2.00 to +5.00 | Blurry at near and sometimes distance |
| High Farsighted | Above +5.00 | Significant blur at all distances without correction |
The NEI notes that high myopia (–6.00 or greater) significantly increases lifetime risk of retinal detachment, glaucoma, and myopic macular degeneration — which is why people with high prescriptions need more frequent monitoring than the average healthy adult.
CYL (Cylinder): Your Astigmatism Correction
A CYL value means you have astigmatism — your cornea is slightly oval instead of perfectly spherical. CYL measures how much correction compensates for that irregular curve, also in diopters.
CYL can be negative or positive depending on the lab’s convention. Some write astigmatism as minus, others as plus — your optician can convert between the two. A blank CYL means no astigmatism correction needed. CYL of –0.50 is mild. CYL of –2.50 or higher is moderate to significant.
AXIS: The Direction of Your Astigmatism
AXIS only appears when there’s a CYL value — they always travel together. It’s a number from 1 to 180, indicating the angle at which the cylindrical correction must be oriented in the lens.
AXIS 90 means the correction runs vertically. AXIS 180 means horizontally. Each degree matters — a lens cut to the wrong AXIS distorts vision rather than correcting it. This is one reason you can’t order contacts using a glasses prescription: the orientation information must be verified separately for a lens sitting on the cornea.
ADD: Your Reading Boost (Progressive/Bifocal Prescriptions)
ADD only appears when you need different power for near vision — most commonly in people over 40 dealing with presbyopia, the age-related stiffening of the eye’s natural lens that makes close focus increasingly difficult. The AAO notes that virtually everyone experiences some degree of presbyopia by their mid-40s.
ADD is always a positive number, usually between +1.00 and +3.00. It represents extra magnifying power added on top of your distance prescription for near objects. Both eyes typically get the same ADD value.
If there’s an ADD on your prescription, you’ll likely need bifocal, trifocal, or progressive lenses.
PD (Pupillary Distance): The Number Doctors Sometimes Skip
PD is the distance between the centers of your pupils in millimeters — typically 54–74mm for adults. It tells the optician exactly where to center the optical correction in each lens.
Some prescriptions give a single PD (like 63mm) combining both eyes. Others list separate monocular measurements (like 31.5 / 31.5) for greater precision.
Why it matters: Online glasses retailers need your PD to cut your lenses correctly. If your doctor didn’t include it on the printed prescription, you can measure it yourself with one of several smartphone apps, or simply call the office — they’re required to provide it in most states. Don’t let anyone tell you it’s unavailable.
Never order contacts using a glasses prescription. Contact lens prescriptions include additional measurements: base curve (the curvature of the back of the lens), diameter, and brand/material — all calibrated for a lens sitting on the corneal surface. The power values may differ slightly from your glasses Rx because contacts sit approximately 12mm closer to the eye. Always get a separate contact lens exam and fitting from your optometrist.
What “20/40 Corrected to 20/20” Actually Means
Visual acuity — the 20/20 notation — isn’t on your prescription, but your doctor may reference it. The first number is the testing distance (20 feet). The second is how that line compares to normal vision.
20/40 means you can read at 20 feet what a typical eye can read at 40 feet. Functionally poor without glasses.
Corrected to 20/20 means with your prescribed lenses, you can read the standard 20-foot line. Many people correct to 20/15 or better with the right prescription.
For driving, most states require at least 20/40 vision in your better eye — with or without correction.
Glasses vs. Contact Lens Prescriptions
Related but not identical:
- Power may differ slightly at higher prescriptions — contacts sit 12mm closer to the eye than glasses, which changes effective lens power
- Contact prescriptions have extra parameters — base curve (BC), diameter (DIA), brand, and material type
- Contact prescriptions expire separately — typically after one year, because a contact lens fit also monitors corneal health over time
If you’ve only worn glasses and want to try contacts, schedule a separate contact lens exam — usually $50–$100 added to a routine exam.
A Prescription Decoded
Here’s a realistic example:
| OD (Right) | OS (Left) | |
|---|---|---|
| SPH | –2.50 | –3.00 |
| CYL | –0.75 | –1.00 |
| AXIS | 175 | 010 |
| ADD | +2.00 | +2.00 |
Reading it:
- Moderately nearsighted in both eyes (–2.50 and –3.00)
- Mild astigmatism in both eyes (–0.75 and –1.00)
- Astigmatism runs nearly horizontally in the right eye (175°) and nearly vertically in the left (10°)
- Needs reading power (+2.00), suggesting over 40 — these glasses will be progressives or bifocals
- The eyes are meaningfully different from each other, and both are very different from any generic reading glasses off a rack
Keep a photo of your prescription on your phone. Knowing what the numbers mean makes every conversation with an optician, an online retailer, or a new eye doctor more useful.