How to Read Your Eye Prescription: SPH, CYL, Axis, and Everything Else Explained
Your doctor hands you a prescription card, and it looks like a row of numbers and abbreviations that mean nothing at a glance. That card is everything you need to order prescription glasses online, but only if you know what each field actually means. The numbers are straightforward once you know the logic behind them.
Quick Reference: Your Prescription at a Glance
Field | What It Means | Example | How to Read It |
OD | Right eye (oculus dexter) | OD: -2.00 | Your right eye needs a -2.00 SPH lens |
OS | Left eye (oculus sinister) | OS: -1.75 | Your left eye needs a -1.75 SPH lens |
SPH | Lens power to correct nearsight or farsight | -2.00 / +1.50 | – = nearsighted; + = farsighted; higher number = stronger |
CYL | Astigmatism correction strength | -0.75 | Blank = no astigmatism; any value = correction needed |
Axis | Direction of astigmatism (1–180°) | 090 | Only appears with CYL; copy exactly as written |
ADD | Extra reading power for bifocals/progressives | +2.00 | Always positive; applies to the reading zone of the lens |
PD | Distance between pupils in mm | 63 / 31/32 | Single number = both eyes; two numbers = right/left separately |
Prism | Corrects eye alignment issues | 1.5 BU | Number = strength; BU/BD/BI/BO = direction of correction |
What Is SPH?
SPH stands for sphere. It tells you how strong your lenses need to be to bring your vision into focus, and whether you are nearsighted or farsighted.
How to Read the Sign
● Minus (–) means nearsighted (myopia). You can see close up but struggle with distance. Nearsighted glasses carry a negative SPH value.
● Plus (+) means farsighted (hyperopia). You see better at a distance but struggle up close.
How to Read the Number
The number is measured in diopters (D), increasing in 0.25 steps. The further from zero, the stronger the prescription.
SPH Value | What It Means |
0.00 to –1.00 | Mild nearsightedness |
–1.25 to –3.00 | Moderate nearsightedness |
–3.25 to –6.00 | High nearsightedness |
–6.00 and above | Very high nearsightedness |
+0.25 and above | Farsightedness |
If your SPH field says pl or plano, it means zero, no correction needed in that eye for distance.
What Is CYL?
CYL stands for cylinder. It measures the degree of astigmatism in your eye, which happens when the cornea or lens is not perfectly round.
A CYL value on your glasses prescription means your lenses need an extra curve in a specific direction to correct blurry or distorted vision. Not everyone has astigmatism. If your CYL field is blank or reads 0.00, your eyes are symmetrical enough that no astigmatism correction is needed.
Positive vs. Negative CYL
Prescriptions can be written in two formats: minus cylinder (most common in the US) or plus cylinder (used in some other countries). The two formats describe the same correction differently. If you are ordering astigmatism glasses online, use the format written on your prescription without converting it.
CYL Always Comes with an Axis
CYL on its own is not enough. The lens needs to know not just how much correction to apply, but in which direction. That is what Axis does.
What Is Axis?
Axis tells the lab where to position the cylinder correction on your lens. It is a number from 1 to 180, representing degrees of orientation.
Axis only appears when CYL has a value. If your CYL is blank, Axis will be blank too.
What the Numbers Mean
The Axis number does not indicate severity. An Axis of 180 is not worse than an Axis of 45. It simply tells the lens maker which angle to cut the astigmatism correction at.
Axis Range | Orientation |
Around 180 or 0 | Horizontal astigmatism |
Around 90 | Vertical astigmatism |
Around 45 or 135 | Oblique astigmatism |
Getting the Axis wrong, even by a few degrees, can cause noticeable distortion. Always copy the number exactly as written on your prescription.
What Are the Other Numbers on Your Prescription?
ADD
ADD stands for addition power. It appears on prescriptions for bifocal or progressive lenses, and tells the lab how much extra magnification to add to the bottom portion of the lens for reading.
● ADD values are always positive, typically between +0.75 and +3.00
● Both eyes usually carry the same ADD value
● If you have a single-vision prescription for distance only, ADD will be blank
PD (Pupillary Distance)
PD is the distance in millimeters between the centers of your two pupils. It tells the lab where to center the optical prescription on each lens.
PD comes in two formats:
● Single PD: one number for both eyes together, e.g., 64mm
● Dual PD: two numbers, one per eye, e.g., 32/32. The first number is the right eye (OD), the second is the left (OS)
PD is not always included on a printed prescription. If yours does not have it, you can measure it at home with a ruler and a mirror, or ask your eye doctor directly. Most online retailers, including Lensmart, require PD to process your order.
Prism
Prism appears on prescriptions when the eyes do not align properly, causing double vision or eye strain. It is expressed as a number (the strength) and a direction abbreviation:
● BU = base up
● BD = base down
● BI = base in
● BO = base out
Most prescriptions do not include prism. If yours does, make sure the retailer supports prism lenses before ordering.
How Do I Order Prescription Glasses Online?
Once you understand your prescription, ordering online is straightforward.
Step 1: Have Your Full Prescription Ready
You need the values for SPH, CYL, Axis (if applicable), ADD (if applicable), and PD. If any field is blank on your prescription, leave it blank when ordering.
Step 2: Select Your Frame
Choose based on fit, face shape, and style. Frame measurements are listed on most product pages. Pay attention to lens width, bridge width, and temple length.
Step 3: Choose Your Lens Type
Lens Type | Who It's For |
Single vision | One prescription throughout the lens (distance or reading) |
Two zones: distance on top, reading on bottom | |
Gradual transition from distance to reading, no visible line | |
Non-prescription | Plano lenses, blue light blocking, or fashion only |
Step 4: Enter Your Prescription
Type each value exactly as written. Double-check CYL sign (+ or –) and Axis number. A small typo here affects how well the lenses work.
Step 5: Upload or Verify
At Lensmart, you can enter your prescription values directly during checkout or upload your prescription card for review.
Prescription Glasses Can Be Stylish Too
A prescription does not limit your frame choices. At Lensmart, hundreds of styles across every shape and aesthetic are available with prescription lenses built in. Cat eye, aviator, minimalist metal, oversized, vintage, whatever fits your face and your look.
Beyond everyday eyeglasses, prescription sunglasses, sports glasses, and kids' glasses are all available too. Single vision, bifocal, progressive, and even prism lenses are supported, so whether your prescription is simple or complex, Lensmart can fill it. The prices are easy on the wallet.
Frequently Asked Questions
What Is the Difference Between OD and OS on My Eye Prescription?
OD is the right eye (oculus dexter) and OS is the left eye (oculus sinister). Always check which row you are reading before entering prescription values. Mixing them up will produce lenses that are correct on paper but wrong for each eye.
Can I Use My Contact Lens Prescription to Order Glasses?
No. Contact lens and glasses prescriptions are not interchangeable. Contact prescriptions include additional measurements like base curve and diameter, and the power values often differ from a glasses prescription. Always use a glasses-specific prescription when ordering eyeglasses.
How Long Is an Eye Prescription Valid?
In most US states, an eyeglass prescription is valid for one to two years from the exam date. Check the expiration date printed on your card. If it has expired, you need a new eye exam before placing an order.
What Does It Mean If My Prescription Has a Prism Value?
Prism corrects eye alignment issues that cause double vision or eye strain. Lensmart supports prism lenses: when entering your prescription at checkout, select "Add Prism" and fill in the prism value and base direction (In, Out, Up, or Down). Both vertical and horizontal prism are supported. If your prescription is complex, reach out to [email protected] before ordering.
