How to Read Your Glasses Prescription: SPH, CYL, AXIS and ADD

Your glasses prescription is a short code that tells a lens maker exactly how to correct your vision. It lists numbers for each eye, such as SPH, CYL and AXIS, and sometimes ADD or prism. Once you know what each column means, it is much easier to understand, and to know what to bring when ordering new glasses.

An example prescription

Prescriptions are usually written as a small table. Here is a made-up example:

In this example, both eyes are short-sighted, both have some astigmatism, and the wearer needs extra help for reading. The layout varies between optometrists, but the terms are the same.

What each part means

R and L, OD and OS

R means right eye and L means left eye. You may also see the Latin terms OD (oculus dexter, right eye) and OS (oculus sinister, left eye). OU means both eyes.

SPH (sphere)

SPH is the main power of the lens, measured in dioptres (D). It corrects short-sightedness or long-sightedness.

• Minus (-) numbers mean short-sighted (myopia): distant things look blurry.
• Plus (+) numbers mean long-sighted (hyperopia): close things are harder to focus on.
• The bigger the number, the stronger the prescription. Stronger prescriptions can mean thicker lenses; see lens materials and thickness.

CYL (cylinder)

CYL corrects astigmatism, which happens when the eye is shaped more like a rugby ball than a soccer ball, so light focuses unevenly. If this column is blank, you may not have astigmatism, or it is too small to correct.

AXIS

AXIS is a number from 1 to 180 that shows the direction of the astigmatism correction, in degrees. It only appears when there is a CYL value. It is not a strength; it is an angle.

ADD (addition)

ADD is extra magnifying power added to the bottom part of the lens for reading and close work. It is common from around middle age, when the eye's natural focusing becomes less flexible. ADD is used for bifocal and progressive lenses, or for separate reading glasses. Learn more in single vision, bifocal and progressive lenses.

Prism and base

Prism is less common. It helps when your eyes do not line up perfectly, which can cause double vision or strain. It is written as a number with a direction called the base (for example, base in, out, up or down).

What is not usually on your prescription

Pupillary distance (PD)

PD is the distance between the centres of your pupils. It is needed to make your glasses, but it is not always written on the prescription. We recommend having it measured properly. Read more in our guide on what pupillary distance is.

Frame and lens choices

Your prescription tells us the correction, not the frame size or lens type. For those, see how to read glasses size numbers and anti-reflective coating.

Glasses prescriptions vs contact lens prescriptions

They are not the same. A contact lens sits on your eye, while glasses sit a short distance in front, so the powers can differ. Contact lens prescriptions also include fitting details like base curve and diameter. You cannot use a contact lens prescription to make glasses.

How long is a prescription valid?

Your optometrist will advise how often you should have your eyes checked. Vision can change over time, and an eye exam is also a check on your eye health, not just your prescription. If things look blurry, you get headaches or you strain to see, it may be time for a check. Our guide on when to update your glasses prescription covers the common signs.

We are not optometrists and do not do eye exams. For a new prescription or any concerns about your eyes, see an optometrist or doctor. You can find general eye health information on Healthify.

Got a new prescription? Visit our Panmure store or learn about prescription updates and same-day glasses. You can also browse our optical frames.