How to Read Your Glasses Prescription — OD, OS, SPH, CYL Explained
A glasses prescription looks like a spreadsheet of abbreviations, but it is really just six pieces of information — two per eye plus one for reading distance, and one for how far apart your pupils sit. Once you can read one line, you can order glasses online with confidence and catch errors before they become expensive. Here is the full breakdown, field by field.
The Two Columns: OD and OS
Prescriptions are written in two columns:
- OD (oculus dexter) = your right eye
- OS (oculus sinister) = your left eye
Some opticians write R and L instead — same thing. Always double-check which column is which when ordering online; swapping them is the single most common ordering mistake.
SPH: Sphere (Your Basic Correction)
SPH tells you how strong your main correction is, in diopters (D).
- Negative number (e.g. -2.50) = nearsighted (myopia) — things far away are blurry
- Positive number (e.g. +1.75) = farsighted (hyperopia) or age-related reading difficulty
- Zero or blank = no spherical correction needed for that eye
The further the number is from zero, the stronger the lens. This is the number that determines whether you should choose a thin high-index material: beyond about -4.00, standard 1.50 plastic lenses get noticeably thick at the edges.
CYL and AXIS: Astigmatism
CYL (cylinder) corrects astigmatism — where the cornea is shaped more like a rugby ball than a basketball, so different directions focus differently.
- CYL is negative in most prescriptions (-0.75, -1.25, etc.). Higher numbers = more astigmatism.
- AXIS is a number from 1 to 180 — it is an angle, not a strength. It tells the lab which direction to grind the cylinder correction.
If there is no CYL value for an eye, you have no astigmatism there. If CYL exists, AXIS must exist — a prescription with CYL but no AXIS is incomplete, not a typing error to ignore.
ADD: The Reading Addition
ADD appears only for progressive, bifocal and reading prescriptions. It is the extra magnifying power added at the bottom of the lens for close work, usually +1.00 to +3.00, and it is almost always the same for both eyes.
No ADD value means the prescription is single-vision — one distance for everything. If you are over 40 and struggling with tiny print, but your prescription has no ADD, ask whether you need a reading addition; that conversation is worth one appointment.
PD: Pupillary Distance
PD is the distance between your pupils in millimeters — the one number that is often left off the prescription because it is technically optical fitting data, not a medical measurement.
You cannot skip it when ordering online: lenses must be centered on your pupils or you will experience distortion, eye strain and headaches, especially at higher prescriptions. If your PD is missing, you can measure it at home with a mirror and a ruler (or a phone app), or ask the store where you were examined — they usually provide it on request.
Typical adult PD: 54-74 mm. Many prescriptions list a single number (e.g. 63); some list two (one per eye, e.g. 31.5/31.5) — both are fine for ordering.
A Worked Example
Take this typical prescription:
| SPH | CYL | AXIS | ADD | |
|---|---|---|---|---|
| OD (right) | -3.25 | -0.75 | 90 | — |
| OS (left) | -3.00 | -1.25 | 85 | — |
Reading it: moderately nearsighted in both eyes (-3.25 and -3.00), with mild astigmatism corrected at near-vertical angles (90 and 85 degrees). Single vision — no reading addition. A good lens choice here is 1.60 high-index material to keep edges slim without paying for 1.74, and any frame width that fits the measured temple span.
Common Mistakes When Ordering Online
- Swapping OD and OS. Verify the column headers twice before checkout.
- Copying the CYL sign wrong. Most prescriptions use negative cylinders; some countries write positive. Do not convert signs yourself — enter exactly what is written.
- Rounding the SPH. Enter values in 0.25 steps exactly as prescribed; do not "simplify" -3.25 to -3.00.
- Ignoring the expiry date. Prescriptions are typically valid 1-2 years; a prescription written after a big vision change should be re-checked before ordering online.
- Skipping PD. Always confirm it is on your order before paying.
FAQ
Can I use my prescription to buy glasses anywhere? Yes — a prescription is yours, and online retailers will ask you to upload or enter it. In many regions the seller must verify it, so having a photo of the signed prescription is useful.
What does "plano" mean? Zero correction — the same as 0.00 in the SPH column.
My two eyes have very different numbers. Problem? Not necessarily, but a large difference (over ~2.00 diopters) is called anisometropia and is worth mentioning to your optician when choosing frame size and lens material.
The Bottom Line
SPH is your main correction, CYL and AXIS handle astigmatism, ADD is for reading, and PD centers the lenses on your eyes. Copy the numbers exactly as written, never swap the eyes, and use the strength of SPH to decide whether you need high-index lenses. Once you can read your own prescription, ordering glasses online stops being a gamble.