How to Read Your Glasses Prescription: SPH, CYL, Axis and More

You leave the optometrist's practice with a slip of paper covered in abbreviations, plus and minus signs and numbers with two decimal places. For most people it might as well be written in code. Yet your glasses prescription is a precise, logical description of how your eyes focus light, and once you understand the handful of terms it uses, it becomes easy to read.

Understanding your prescription helps you compare quotes, check that new glasses have been made correctly, see how your eyes are changing over time and have a more informed conversation with your eye care professional. This guide explains every part of a standard prescription, what the numbers tell you about your vision, and why a glasses prescription cannot simply be used to order contact lenses.

A typical prescription at a glance

Prescriptions are usually laid out as a small table, with one row for each eye. Layouts differ between countries and practices, but the information is the same. Here is an example:

In plain language, this person is moderately short-sighted in both eyes, has a small amount of astigmatism in each eye, and needs an extra +2.00 for reading, which is typical of someone in their fifties. No prism is prescribed. Below we explain each column.

OD, OS and OU: which eye is which

The abbreviations come from Latin: OD (oculus dexter) is the right eye, OS (oculus sinister) is the left eye, and OU (oculus uterque) means both eyes. Many prescriptions now simply say R and L or Right and Left. The right eye is conventionally listed first. Remember that it is your right eye, not the right-hand side of the page as the examiner sees you.

SPH: sphere

The sphere value describes the main focusing error of the eye, measured in dioptres (D). A dioptre is a unit of lens power; the larger the number, the stronger the lens.

• A minus sign (-) means short-sightedness (myopia): distant objects are blurred. The lens diverges light.
• A plus sign (+) means long-sightedness (hyperopia): the eye has to work harder to focus, especially up close. The lens converges light.
• Plano, PL, 0.00 or the infinity symbol means no spherical correction is needed.

Powers are almost always written in steps of 0.25 D. As a rough guide, up to about 3 D is often described as low, 3 to 6 D as moderate, and above 6 D as high, though definitions vary. High myopia is associated with an increased risk of some eye conditions, which is why people with strong minus prescriptions are advised to have regular eye health checks.

CYL and AXIS: astigmatism

If the front surface of the eye (the cornea) or the lens is shaped more like a rugby ball than a football, light is focused differently in different directions. This is astigmatism, and it is corrected by a cylindrical lens component.

Cylinder (CYL)

The CYL value shows how much astigmatism there is, in dioptres. If the CYL box is empty, or says DS (dioptre sphere) or SPH, you have no significant astigmatism. The cylinder can be written with a minus sign or a plus sign. Ophthalmologists sometimes use plus cylinder notation, while most optometrists use minus cylinder notation. Both describe exactly the same correction; they are simply two ways of writing it, and an optician can convert between them.

Axis

The axis is always written with the cylinder and shows the orientation of the astigmatism correction. It is an angle between 1 and 180 degrees, where 90 means vertical and 180 means horizontal. The axis is not a measure of strength: an axis of 180 is not worse than an axis of 10. It simply tells the lab how to rotate the cylinder in the frame. Because of this, a small error in the axis of a strong cylinder can noticeably blur vision, which is why accurate measurement and lens mounting matter.

ADD: reading addition

ADD stands for addition, the extra plus power added to the distance prescription for near vision. It appears when you have presbyopia, the normal age-related loss of near focusing, which usually becomes noticeable in the forties. The ADD is usually the same for both eyes and typically ranges from about +0.75 to +3.00.

The ADD is used in three ways. For bifocal or progressive lenses, the lab builds it into the lower part of the lens. For single-vision reading glasses, the ADD is added to the sphere: in our example, -2.25 plus +2.00 gives a reading sphere of -0.25 for the right eye, with the same cylinder and axis. Some prescriptions also give an intermediate add for computer glasses. If you are unsure, ask your optometrist to write out the reading prescription separately. Our guide to lens types explains bifocals and progressives.

PRISM and BASE: eye alignment

Most prescriptions do not include prism. When present, prism helps people whose eyes do not align perfectly, for example in some cases of double vision or binocular vision problems. A prism lens does not change focus; it shifts the image so the two eyes can work together more comfortably.

Prism is measured in prism dioptres (written as a triangle symbol or as PD, Δ or p.d.) and always has a base direction: BU (base up), BD (base down), BI (base in, towards the nose) or BO (base out, towards the ear). For example, 2Δ BI means two prism dioptres with the base towards the nose. If you have prism in your prescription, it is especially important that glasses are made by a reliable lab and checked carefully. Prism in glasses does not cure an eye turn, and exercises do not replace it, although some binocular problems such as convergence insufficiency may be treated with supervised vision therapy.

PD: pupillary distance

The PD is the distance between the centres of your pupils, measured in millimetres. It is not a measure of your vision but a measurement used to position the optical centres of the lenses in front of your eyes. It may be written as one number, such as 63, or as two monocular values, such as 31.5/31.5, measured from the centre of the nose to each pupil. Some practices include it on the prescription; others record it at the time of dispensing. Our guide to pupillary distance explains why it matters, especially for progressive lenses.

Other information you may see

• VA (visual acuity): the sharpness of vision achieved with the prescription, for example 6/6 or 20/20.
• BVD or vertex distance: the distance between the back of the lens and the eye, important for strong prescriptions.
• Intermediate (INT): a power for computer or arm's-length distance.
• Remarks: recommendations such as anti-reflective coating, tint, or advice about driving.
• Date and expiry: prescriptions are typically valid for one to two years, depending on country and age.

What your prescription does not tell you

A prescription describes the optical correction your eyes need, but it says nothing directly about eye health. Two people with identical prescriptions can have very different eyes: one may have early glaucoma or a cataract and the other perfectly healthy eyes. This is why a full examination includes checks of the eye pressure, optic nerve and retina in addition to the refraction. Our page on eye examinations explains what a complete check involves.

Nor does a prescription say how well you see without glasses. Visual acuity charts measure that separately. If you are curious, you can try our online vision tests, but they are a screening aid and cannot replace a professional examination.

Glasses prescription versus contact lens prescription

A common mistake is to assume that a glasses prescription can be used to buy contact lenses. It cannot, for several reasons:

1. Vertex distance: glasses sit about 12 mm in front of the eye, while contact lenses sit on the eye. For prescriptions stronger than about ±4.00 D, the effective power changes significantly, so the contact lens power differs from the glasses power.
2. Fit parameters: contact lens prescriptions include base curve (BC) and diameter (DIA), which describe how the lens fits the curvature of your cornea.
3. Brand and material: the specific lens type matters for oxygen supply, comfort and safety.
4. Astigmatism handling: toric contact lenses come in limited cylinder and axis steps, and small amounts of astigmatism are sometimes left uncorrected or corrected differently.
5. Health assessment: a contact lens fitting checks that the lenses suit the health of your cornea and tear film.

Read more about fittings, lens types and safe wear in our contact lenses guide.

Why prescriptions change

It is normal for prescriptions to change over time. Myopia often increases during childhood and adolescence. The reading addition increases gradually between about 40 and 60. Astigmatism can shift slightly with age. However, sudden or large changes deserve attention: rapidly increasing myopia in an adult can occur with developing cataract, and fluctuating vision can be caused by changing blood sugar levels in diabetes. If your new prescription is very different from your last one, ask your optometrist why.

Comparing old and new prescriptions can also help you understand your glasses. Small changes of 0.25 D are common and may not always require new glasses, while changes of 0.50 D or more, or new astigmatism, are more likely to make a noticeable difference.

Checking your new glasses

When you collect new glasses, the optician should verify the lenses against the prescription using a lensmeter. If after a week or two of wear you still have blurred vision, headaches, a feeling of the floor tilting or one eye straining, return for a check. Possible causes include an incorrect axis, wrong optical centring, incorrect PD or a frame that sits poorly. Most problems are easily corrected.