How to Read Your Glasses Prescription
Those numbers and abbreviations on your prescription slip aren't as mysterious as they look. Here's exactly what each one means — and why it matters when choosing your next pair.
By Eddie Coyle Optometrists · Glasgow
Everything you need to order a pair of glasses.
The Slip That Confuses Everyone
You've just had your eye examination, and you're handed a small piece of paper covered in abbreviations, minus signs, and decimal numbers. If you've ever held that slip and wondered what any of it actually means, you're in excellent company — it's one of the most common questions we hear in practice. This guide will walk you through every element, line by line. Understanding your prescription isn't just satisfying — it helps you make better decisions about your eyewear, spot errors before they become problems, and have more informed conversations with your optometrist at your next visit.
OD and OS: Left Eye, Right Eye
Your prescription is divided into two rows: OD and OS. These are Latin abbreviations — oculus dexter (right eye) and oculus sinister (left eye). OD is always listed first. Some prescriptions also include OU (oculus uterque) — meaning both eyes — particularly when a value applies equally to each.
| Abbreviation | Meaning |
|---|---|
| OD | Right eye (oculus dexter) |
| OS | Left eye (oculus sinister) |
| SPH | Sphere — the main focusing correction |
| CYL | Cylinder — correction for astigmatism |
| AXIS | Direction of astigmatism (1–180°) |
| ADD | Reading addition (for bifocals / varifocals) |
| PD | Pupillary distance — not always on this slip |
Sphere (SPH): The Headline Number
The sphere value tells you the primary focusing power of the lens required. It's measured in dioptres (D) and given to two decimal places — for example, −2.25 or +1.00.
A minus (−) sign means you are myopic — short-sighted.
Objects in the distance appear blurred, and a concave (thinner at the centre) lens is used to correct this. The higher the negative number, the stronger the correction required.
A plus (+) sign means you are hyperopic — long-sighted.
Your eye has to work harder to focus at all distances, and a convex (thicker at the centre) lens is needed. It's worth noting that low levels of hyperopia are common in children and young adults, whose eyes can compensate naturally — you may only notice symptoms such as headaches or eyestrain.
If the sphere field reads Plano or 0.00, it means no spherical correction is needed in that eye — though the prescription may still include a cylinder value for astigmatism.
Cylinder (CYL) and Axis: Correcting Astigmatism
Not everyone has a cylinder value — if these fields are blank or read 'DS' (dioptre sphere), you don't have significant astigmatism. But for those who do, these two figures always appear together. Astigmatism occurs when the cornea or lens is shaped more like a rugby ball than a football — with different curvatures in different meridians. This causes light to focus unevenly, resulting in blur and distortion at all distances.
The CYL value tells the lens manufacturer how much additional power is needed in a particular direction. Like sphere, it can be negative or positive depending on the notation convention your optometrist uses (both describe the same thing — some practices use 'minus cylinder', others 'plus cylinder').
The Axis is a number between 1 and 180, representing the angle (in degrees) of the principal meridian. Without it, the cylinder correction would have no orientation — the two values are meaningless apart from each other. An axis of 180 and an axis of 90 describe astigmatism in completely different directions.
Add (Addition): For Reading and Varifocals
If you're over 40 and finding your arms aren't quite long enough to hold the menu at a comfortable reading distance, you're experiencing presbyopia — the gradual stiffening of the eye's natural lens that reduces its ability to change focus.
The ADD value corrects for this.
ADD is always a positive number (typically between +0.75 and +3.50) and is usually the same for both eyes. It represents the additional power needed on top of your distance prescription to bring near objects into focus. It's used in reading glasses, bifocals, and the near zone of varifocal lenses.
Understanding the numbers on your prescription is the first step to finding the right lenses for your lifestyle.
A Note on Pupillary Distance (PD)
Your PD — the distance in millimetres between the centres of your pupils — is essential for centring your lenses correctly within your frame.
Without an accurate PD, even a perfect prescription will cause unnecessary eye strain. PD is sometimes listed on your prescription slip, and sometimes it isn't.
Legally in the UK, optometrists are required to provide your prescription upon request, but PD is treated separately. If
you plan to order glasses online, ask your optometrist to include it — or see our dedicated guide to understanding and measuring PD.
Ready for your next eye examination?
Book online or call our Dennistoun or Cambuslang practice.
Bring any previous prescription slips — they help us track changes over time and give you a fuller picture of your eye health.
