How to Read Your Glasses Prescription: SPH, CYL, AXIS, and PD Explained
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Your eye doctor hands you a piece of paper covered in abbreviations and numbers. SPH. CYL. AXIS. PD. Maybe ADD. Your eyes glaze over. You nod, smile, and pretend you understand what any of it means.
You're not alone. Most people have no idea what their prescription actually says — and because of that, they feel helpless when trying to order glasses online. Let's fix that.
What Is a Glasses Prescription?
A prescription is a set of instructions for lens makers: "Grind the lenses this way so this person's vision improves." Each number tells the lens maker something specific about your eye and how to correct it. You need it to order glasses online — and once you understand the jargon, it takes about 30 seconds to read.
SPH (Sphere) — Your Main Correction
This is your primary prescription strength. It corrects nearsightedness (myopia) or farsightedness (hyperopia).
Negative numbers (like -2.50) mean you're nearsighted — you see things up close clearly, but distance is blurry. Positive numbers (like +1.75) mean you're farsighted — distance is okay but reading up close is harder. The bigger the number in either direction, the stronger the correction needed.
Most people have an SPH between -6.00 and +6.00.
CYL (Cylinder) — Astigmatism Correction
This number corrects astigmatism — a condition where your cornea is slightly irregular, causing blurred vision at all distances. About 60% of people have some degree of astigmatism.
If your CYL field is blank or says "PL" (plano), you don't have astigmatism. If it's filled in, CYL is always applied in addition to your SPH — your lens needs both corrections ground into it.
AXIS — The Direction of Your Astigmatism
AXIS only matters if you have CYL. It tells the lens maker at what angle to apply the cylinder correction — think of it as the direction on a clock face (1–180 degrees) where the astigmatism correction needs to go.
This is precise. If your AXIS is 90, it's 90 — not 85, not 95. Enter it exactly as written on your prescription.
PD (Pupillary Distance) — Where to Centre the Lens
PD is the distance between the centres of your pupils, measured in millimetres. It tells the lens maker where to position the optical centre of each lens so you're looking through the most accurate part.
PD is usually between 54 and 74mm. Frustratingly, many optometrists don't include it on your written prescription — but you can measure it yourself at home in about 30 seconds:
- Stand ~20cm from a bathroom mirror with a ruler held across your nose bridge.
- Close your right eye. Align the ruler's zero mark with the centre of your left pupil.
- Open your right eye, close your left. Read where the centre of your right pupil falls.
- That number is your PD.
We have a full step-by-step PD guide here →
ADD (Addition) — Progressive Lens Power
You'll only see ADD if you need progressive lenses. It indicates how much extra magnification you need for close-range reading, in addition to your distance correction. ADD values typically run from +0.75 to +3.00. If it's not on your prescription, you don't need progressives.
Reading a Complete Prescription — A Real Example
| SPH | CYL | AXIS | ADD | PD | |
|---|---|---|---|---|---|
| OD (Right eye) | -2.50 | -1.00 | 90 | — | 63 |
| OS (Left eye) | -2.75 | -0.75 | 85 | — | — |
Translation: your right eye is nearsighted (-2.50) with astigmatism (-1.00 at 90°). Your left eye is slightly more nearsighted (-2.75) with less astigmatism (-0.75 at 85°). Pupil distance is 63mm. That's everything a lens maker needs — and now you understand every part of it.
A Few Other Things You Might See
OD = right eye (oculus dexter). OS = left eye (oculus sinister). Some prescriptions use R and L instead. Prism & Base appear only if you have an eye alignment issue — most people don't. If you do, include it when ordering.
How to Order Online With Your Prescription
- Write down (or photograph) your SPH, CYL, AXIS, PD, and ADD for each eye.
- Choose your frames.
- Select your lens type — single vision for one distance, progressive if you need both near and far.
- Enter your numbers into the order form exactly as written.
- Add any coatings — blue light filter is $9.99.
- Order. Done.
At Lenstrove, this process takes about five minutes.
The Takeaway
Your prescription isn't magic. SPH tells the lens maker how strong to make your lens. CYL and AXIS add astigmatism correction at the right angle. PD centres the lens. That's the whole thing.
Once you understand this, ordering glasses online is genuinely simple — and dramatically cheaper than going through a retail optician.