Eye prescriptions are written in a standardized format that can feel like a foreign language — SPH, CYL, AXIS, ADD, OD, OS. This tool decodes each value in plain English so you understand exactly what your optometrist prescribed and why. Enter your prescription values below for a complete explanation.
OD — Right Eye
OS — Left Eye
PD (Pupillary Distance)
Your Prescription Summary
Right Eye (OD) Breakdown
Left Eye (OS) Breakdown
Pupillary Distance (PD)
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Enter your prescription values to get a full explanation
Quick Reference: What Each Value Means
| Value | Full Name | What It Corrects | Range |
|---|---|---|---|
| SPH | Sphere | Nearsightedness (−) or Farsightedness (+) | −20 to +20 |
| CYL | Cylinder | Astigmatism (irregular cornea) | −6 to +6 |
| AXIS | Axis | Orientation of astigmatism correction | 1–180° |
| ADD | Addition | Reading (presbyopia, bifocals/progressives) | +0.75 to +3.50 |
| PD | Pupillary Distance | Optical center alignment | 55–75mm |
How to Read Your Eye Prescription
Your eyeglass prescription is a standardized document that contains specific measurements for correcting your vision. Understanding each value helps you communicate with your optician, verify online orders, and understand your eye health.
Step 1: Identify OD and OS
OD stands for oculus dexter (Latin for "right eye") and OS stands for oculus sinister (left eye). Some prescriptions use RE (right eye) and LE (left eye) instead. The right eye is always listed first.
Step 2: Understand SPH (Sphere)
The SPH value is the most important number. Negative SPH (e.g., −2.50) means you are nearsighted — you see close objects clearly but distance is blurry. Positive SPH (e.g., +1.75) means farsighted. The larger the number (ignoring the sign), the stronger the correction needed. Values below ±1.00 are mild; ±3.00 and above are strong.
Step 3: Read CYL and AXIS (Astigmatism)
If you have astigmatism, your cornea has an irregular curve like a football rather than a sphere. The CYL value measures how much correction is needed, and AXIS tells the lens maker the angle at which to apply it. Both values always appear together. If your prescription has no CYL entry, you have little or no astigmatism.
Step 4: Check ADD (Reading Addition)
ADD appears on prescriptions for adults typically over 40. As eyes age, they lose the ability to focus on near objects (called presbyopia). The ADD value (always positive, typically +0.75 to +3.00) is added to your distance SPH to create the reading zone in bifocal or progressive lenses.
Step 5: Find Your PD
Pupillary distance (PD) is measured in millimeters between the centers of your pupils. The average adult PD is 60–65mm. This measurement is critical for online orders — an incorrect PD causes the optical center to be misaligned with your pupils, causing eye strain and headaches.
Ordering Glasses Online with Your Prescription
Online eyeglass retailers need: OD SPH, OD CYL, OD AXIS, OS SPH, OS CYL, OS AXIS, and your PD. If you need reading glasses or progressives, they also need the ADD value. Always enter values exactly as written on your prescription — do not round or estimate. Most online retailers accept prescriptions from the past two years.
FAQ
Is this eye prescription explainer free?
Yes, completely free with no signup required. All calculations run in your browser and your prescription data is never sent to any server.
Is my prescription data private?
Yes. Everything runs locally in your browser. No prescription data is transmitted to or stored on any server. Your optical information remains entirely on your device.
What does SPH (Sphere) mean on an eye prescription?
SPH stands for Sphere and indicates the amount of lens power needed to correct nearsightedness (myopia) or farsightedness (hyperopia). A negative SPH value (e.g., -2.50) means nearsighted — you see near clearly but distance is blurry. A positive SPH value (e.g., +1.75) means farsighted — distance may be clear but near vision is blurred.
What does CYL (Cylinder) mean?
CYL stands for Cylinder and indicates the amount of lens power needed to correct astigmatism — an irregular curvature of the cornea or lens that causes blurring at all distances. The CYL value is always accompanied by an AXIS value. Many people have no astigmatism and will see no CYL entry on their prescription.
What is the AXIS number on my prescription?
AXIS is a number from 1 to 180 degrees that specifies the orientation of the cylinder correction for astigmatism. It tells the lens maker the angle at which to apply the cylindrical power. AXIS is only present when there is a CYL value.
What does ADD mean on a prescription?
ADD (Addition) is the magnifying power added to the bottom of a bifocal or progressive lens for reading. It is always a positive number, usually between +0.75 and +3.00. ADD is most common in prescriptions for adults over 40 who have presbyopia — the natural loss of near focus with age.
Can I use this to order glasses online?
This tool helps you understand your prescription, but online retailers need the exact values directly from your written prescription. You will need: OD and OS SPH, CYL, and AXIS values, and your PD (pupillary distance). Some retailers also ask for ADD if you need bifocals or progressives. Always use your written prescription from a licensed optometrist.
What is PD (Pupillary Distance)?
PD stands for Pupillary Distance — the measurement in millimeters between the centers of your pupils. It tells the lens maker where to position the optical center of each lens. Single PD is measured across both eyes. Dual PD is listed as two numbers (e.g., 32/31) for right and left eye separately.