After an eye test, your optometrist will hand you a copy of your prescription. For many people, the sheet of abbreviations and numbers means very little at first glance. Understanding what it all means is important — particularly if you want to order glasses online, where you’ll be entering those values yourself.
This guide explains every part of a standard UK glasses prescription in plain English.
What Your Prescription Looks Like
A standard UK glasses prescription is laid out as a table. It has two rows — one for each eye — and several columns. The right eye is usually listed first, often labelled R or OD (from the Latin oculus dexter, meaning right eye). The left eye is labelled L or OS (oculus sinister, left eye).
The columns you’ll see are:
- SPH (Sphere)
- CYL (Cylinder)
- Axis
- ADD (Addition) — only if you need reading or multifocal lenses
- Prism — only if you have a muscle imbalance in your eyes
Your pupillary distance (PD) may also appear on the prescription, though some opticians note it separately or don’t include it at all.
SPH — Sphere
SPH indicates the main power of your lens — how much correction your eye needs for short-sightedness (myopia) or long-sightedness (hyperopia).
- A negative number (e.g. –2.00) means you are short-sighted: you can see near objects clearly but struggle with distance.
- A positive number (e.g. +1.50) means you are long-sighted: close objects appear blurry.
SPH is measured in dioptres (D) and typically written in increments of 0.25. The higher the number (regardless of whether it is positive or negative), the stronger the prescription.
If the SPH column shows Pl or ∞, this means plano — no sphere correction is needed for that eye.
At Glasses Store, prescriptions with an SPH of +/–4 or above combined with a CYL of +/–2 or above are considered high prescriptions. These can still be accommodated — see the FAQ on the Glasses Store homepage for further detail on high prescription options.
CYL — Cylinder
CYL refers to the amount of astigmatism correction required. Astigmatism occurs when the cornea (the front surface of your eye) is not perfectly spherical — it is shaped slightly like a rugby ball rather than a football. This causes light to focus unevenly, resulting in blurred or distorted vision at all distances.
CYL values can be positive or negative depending on how your optician writes the prescription (some use plus cylinder notation, others use minus — both are valid ways of expressing the same correction). If the CYL box is empty or shows DS (dioptre sphere), you do not have significant astigmatism.
Axis
The Axis value only appears when there is a CYL value. It describes the orientation of the astigmatism correction — the angle at which the cylinder power needs to be positioned in the lens. Axis is measured in degrees from 1 to 180.
CYL and Axis always go together. An Axis value on its own, without a CYL value, has no meaning. If you have a CYL value, you will always have a corresponding Axis value.
ADD — Addition
The ADD value appears if you need different prescriptions for distance and near vision — most commonly as a result of presbyopia, the age-related loss of the eye’s ability to focus at close distances. Presbyopia typically begins to affect people in their early to mid-40s.
ADD is always a positive number and represents the extra magnifying power added to the bottom portion of a bifocal or varifocal lens for reading. A typical ADD value ranges from +0.75 to +3.50. The same ADD value usually applies to both eyes.
If your prescription includes an ADD value, you will need bifocal or varifocal lenses — or a separate pair of reading glasses.
Prism and Base
Prism correction is prescribed when the two eyes don’t work together perfectly, causing symptoms such as double vision, eye strain, or headaches. A prism in the lens bends light without altering its focus, helping the eyes to work together more efficiently.
Prism is measured in prism dioptres and is written with a direction — for example, 2.00Δ Base In or 1.50Δ Base Down. Not all prescriptions include prism correction; it is relatively uncommon.
VA — Visual Acuity
VA stands for visual acuity — a measure of how clearly you can see, typically tested at six metres with your glasses on (if applicable). It is written as a fraction, such as 6/6 (normal vision) or 6/12 (seeing at six metres what someone with normal vision can see at twelve). VA is informational and doesn’t affect how your lenses are made; you don’t need to enter it when ordering glasses online.
PD — Pupillary Distance
Your PD is the distance in millimetres between the centres of your pupils. It ensures the optical centre of each lens aligns with your pupils — which is important for comfortable, accurate vision.
PD may appear as a single number (e.g. 64mm, a monocular binocular measurement) or as two numbers (e.g. R 32 / L 32, giving each eye separately). Both are valid.
Some UK opticians don’t routinely include PD on the prescription — they may measure it separately when dispensing glasses in-store. If your PD isn’t on your prescription, you can ask your optician for it, or measure it yourself. Our guide to what PD is and why it matters when ordering glasses online explains how to do this accurately.
BVD — Back Vertex Distance
BVD is the distance between the back surface of the lens and the front of the eye. It only matters for stronger prescriptions (typically SPH above ±5.00) and is used to ensure the lens power is calculated correctly for the actual distance the lens will sit from your eye. If your BVD is noted, you don’t need to enter it when ordering online — just include it if the order form asks for it.
An Example Prescription
| SPH | CYL | Axis | ADD | |
|---|---|---|---|---|
| R (OD) | –1.75 | –0.50 | 180 | +2.00 |
| L (OS) | –2.00 | –0.75 | 170 | +2.00 |
In the example above: both eyes are short-sighted (negative SPH values), both have mild astigmatism (CYL values), and the ADD of +2.00 means this person needs varifocal or bifocal lenses — or reading glasses in addition to their distance prescription.
Glasses Prescriptions and Contact Lens Prescriptions Are Different
Your glasses prescription and your contact lens prescription are not interchangeable. Contact lens prescriptions include additional measurements (such as base curve and diameter) specific to the lens fitting. Never attempt to use one to order the other.
How Long Is a Prescription Valid?
In most circumstances in the UK, a glasses prescription is valid for two years from the date of issue for adults. Your optician may recommend a shorter validity period — for example, one year — if your prescription is changing quickly, or if you have certain eye health conditions. The validity period should be noted on the prescription itself.
Once your prescription has expired, you’ll need a new eye test before ordering glasses. Read more in our guide on what happens after you order glasses online, which covers the full process from placing your order to delivery.
Ordering With Your Prescription at Glasses Store
When ordering glasses at Glasses Store, you’ll enter your prescription values at checkout. All frames can be fitted with single vision, bifocal, or varifocal lenses and are glazed in-house. If you have questions about your prescription or whether a particular frame is suitable for your prescription strength, the team can advise via the contact page.
Browse the full glasses range to find frames that suit your prescription, face shape, and style.
For further information on optical prescriptions and what they mean, the College of Optometrists and the General Optical Council (the regulator for optometrists and dispensing opticians in the UK) both provide publicly accessible information on eye care standards. Information about who is entitled to a free NHS eye test is available at nhs.uk.
