Understanding the Layout of Your Prescription
An eyeglass prescription is a standardized form that your optometrist or ophthalmologist gives you after an eye exam. While the exact layout varies, all prescriptions contain the same core information.
Here is what a typical prescription looks like:
| SPH | CYL | AXIS | ADD | PD | |
|---|---|---|---|---|---|
| OD (Right) | -2.50 | -0.75 | 180 | +2.00 | 63 |
| OS (Left) | -2.25 | -1.00 | 175 | +2.00 |
Let's break down every column.
OD and OS: Right Eye and Left Eye
These are Latin abbreviations used in medicine:
| Abbreviation | Meaning | Language |
|---|---|---|
| OD | Oculus Dexter — Right Eye | Latin |
| OS | Oculus Sinister — Left Eye | Latin |
| OU | Oculus Uterque — Both Eyes | Latin |
Some modern prescriptions use RE (Right Eye) and LE (Left Eye) instead of OD and OS.
SPH (Sphere): Nearsightedness or Farsightedness
SPH measures the spherical correction needed to fix your distance vision. It is measured in diopters (D).
| SPH Value | Condition | Severity |
|---|---|---|
| 0.00 to -1.00 | Nearsighted (Myopia) | Mild |
| -1.25 to -3.00 | Nearsighted (Myopia) | Moderate |
| -3.25 to -6.00 | Nearsighted (Myopia) | High |
| -6.25 and above | Nearsighted (Myopia) | Extreme |
| +0.25 to +3.00 | Farsighted (Hyperopia) | Mild to Moderate |
| +3.25 and above | Farsighted (Hyperopia) | High |
Key point: The further the number is from zero (in either direction), the stronger your prescription.
When SPH is the only correction on your prescription (no CYL/AXIS), you need single vision lenses — the simplest and most affordable type.
CYL (Cylinder) and AXIS: Astigmatism Correction
CYL and AXIS always appear together. If you have astigmatism, your eye is shaped more like a football than a basketball, and light focuses at multiple points instead of one.
- CYL measures the amount of astigmatism in diopters (typically between -0.25 and -4.00)
- AXIS measures the orientation of the astigmatism in degrees (1-180)
| CYL Value | Astigmatism Severity |
|---|---|
| -0.25 to -1.00 | Mild |
| -1.25 to -2.00 | Moderate |
| -2.25 to -3.00 | High |
| -3.25 and above | Extreme |
If CYL is blank or "SPH" on your prescription, you do not have astigmatism correction. About 30-40% of eyeglass wearers have astigmatism correction in their prescription.
ADD: Reading Magnification
ADD (Addition) is the extra magnifying power added to the bottom portion of a lens for near-vision tasks like reading. It is always a positive number, typically ranging from +1.00 to +3.00 diopters.
ADD appears on your prescription if you need:
- Bifocal lenses — two distinct zones (distance + reading)
- Progressive lenses — gradual transition from distance to reading
- Reading glasses — the ADD value becomes your full prescription for reading
ADD is not needed for single vision distance glasses. It typically appears after age 40 due to presbyopia — the natural loss of near-focus ability.
PD (Pupillary Distance): Essential for Online Orders
PD measures the distance between the centers of your pupils, in millimeters. It tells the lab exactly where to position the optical center of each lens.
| PD Type | What It Measures | Typical Range |
|---|---|---|
| Single PD | Distance between both pupil centers | 54–74 mm |
| Dual PD | Distance from nose bridge to each pupil | 27–37 mm each |
Important: In the US, optometrists are not legally required to include PD on your prescription (it is considered a fitting measurement, not a prescription value). However, PD is required to order glasses online. If your prescription does not include PD, you can measure it yourself — see our guide on How to Measure Pupillary Distance at Home.
Other Abbreviations You Might See
| Abbreviation | Meaning | What It Means for You |
|---|---|---|
| PL / PLANO | Zero correction | No prescription needed for this eye |
| DS | Diopters Sphere | No astigmatism correction (CYL is zero) |
| NV | Near Vision | Prescription for reading only |
| DV | Distance Vision | Prescription for distance only |
| PRISM | Prism correction | Corrects eye alignment issues; measured in prism diopters |
| BASE | Prism direction | The direction of the prism correction (BU, BD, BI, BO) |
| BO | Base Out | Prism oriented outward (toward the ear) |
| BI | Base In | Prism oriented inward (toward the nose) |
| BU | Base Up | Prism oriented upward |
| BD | Base Down | Prism oriented downward |
| BAL | Balance | Non-prescription lens to match the other eye's appearance |
| VA | Visual Acuity | Your clarity of vision (e.g., 20/20, 20/40) |
Reading a Real Prescription: Step-by-Step Example
Let's walk through this actual prescription:
| SPH | CYL | AXIS | ADD | |
|---|---|---|---|---|
| OD (Right) | -2.50 | -0.75 | 180 | +2.00 |
| OS (Left) | -2.25 | -1.00 | 175 | +2.00 |
Reading this prescription:
- Right eye (OD): Nearsighted at -2.50 diopters (moderate myopia), with mild astigmatism of -0.75 at the 180-degree axis, and needs +2.00 reading magnification.
- Left eye (OS): Nearsighted at -2.25 diopters (moderate myopia), with mild astigmatism of -1.00 at the 175-degree axis, and needs +2.00 reading magnification.
- Lens type needed: Progressive or bifocal lenses (because ADD is present — this person needs both distance and reading correction in one pair).
- Alternative: Two pairs — single vision distance glasses (using just the SPH/CYL/AXIS) and separate reading glasses (SPH + ADD for each eye).
Common Mistakes When Reading Your Prescription
- Confusing CYL for SPH — CYL is always paired with AXIS. If there is no AXIS, the number is not CYL.
- Missing the minus sign — -2.50 and +2.50 are completely different corrections. Double-check the sign.
- Assuming ADD is optional — If ADD appears on your prescription and you order single vision lenses, your reading vision will be blurry.
- Using an expired prescription — Most online retailers require a prescription less than 1-2 years old. See our guide on expired prescription rules by state.
- Ignoring prism values — If your prescription includes PRISM, not all online retailers can fulfill it. Check before ordering.
What You Need to Order Glasses Online
To buy prescription glasses online, you need:
- A valid prescription from a licensed eye doctor
- Your PD (pupillary distance) measurement
- Your frame size (or a frame you want to buy from the retailer)
- Your lens type preference (single vision, progressive, bifocal, or reading)
For a complete walkthrough, see our Buying Prescription Glasses Online: Complete Step-by-Step Guide.