A glasses prescription can look more like a code than a clear set of instructions: columns, abbreviations, plus and minus signs, and numbers that seem to change every time you have an eye test. Once you know how to read your glasses prescription correctly, however, it becomes much easier to understand what your lenses are designed to do - and to order glasses with confidence.
Your prescription is unique to your eyes. It is not a measure of how ‘good’ or ‘bad’ your vision is, and it does not tell the whole story of your eye health. Think of it as the precise optical information needed to make lenses that bring your vision into focus.
Start with right and left eye
Most prescriptions have two rows or columns marked R and L, or OD and OS. R/OD refers to your right eye, while L/OS refers to your left eye. OD is short for oculus dexter and OS means oculus sinister - Latin terms still widely used in optometry.
Always check which format a website or order form uses before entering your details. A right and left prescription can differ significantly, so swapping them may leave you with glasses that do not give clear, comfortable vision.
The main prescription values explained
SPH: your spherical power
SPH, or sphere, is the basic lens power required to correct short-sightedness or long-sightedness. It is measured in dioptres and is normally written with a plus or minus sign.
A minus value, such as -2.00, usually indicates short-sightedness. You may see nearby objects clearly but find distance vision - road signs, a classroom board or the television - blurred without correction. A plus value, such as +1.50, generally indicates long-sightedness, where close work may feel more difficult or tiring.
The higher the number away from zero, the stronger the lens power. That does not automatically mean one prescription is more serious than another. Comfort, clarity and eye health are assessed individually during your eye examination.
CYL: correction for astigmatism
CYL, meaning cylinder, is used when you have astigmatism. This is very common and simply means the front surface of the eye, or sometimes the lens inside it, has a shape that causes light to focus unevenly.
Astigmatism can make vision appear blurred, stretched or shadowed at all distances. Your cylinder figure may be written with a minus sign or a plus sign, depending on the prescribing convention used. Both formats can be correct, but they are not interchangeable without converting the full prescription correctly.
If CYL is blank, zero or marked ‘DS’, you do not need a separate astigmatism correction in that eye.
Axis: the direction of astigmatism
If there is a CYL value, there should usually be an Axis value too. This is a number between 1 and 180, such as 90 or 175. It does not represent lens strength. Instead, it tells the glazing team the direction in which the cylinder correction must sit within the lens.
Axis can look like a small detail, but accuracy matters. Entering 18 instead of 180, for example, can affect the clarity of your glasses. Copy it exactly as written.
ADD, prism and other details on your prescription
Some prescriptions contain extra figures because the lenses need to do more than provide one level of distance correction.
ADD: extra help for close vision
ADD, short for addition, is the additional power needed for near tasks such as reading, using a mobile phone or detailed work. It is most commonly included for people who need varifocal, bifocal or reading lenses alongside their distance prescription.
An ADD of +2.00 means that amount is added to the distance prescription in the reading area of the lens. It is usually the same for both eyes, though not always. If your prescription includes ADD, choose a lens type that supports it. Single-vision distance lenses will not provide the reading support your prescriber intended.
Prism: aligning the eyes more comfortably
Prism may be prescribed when the eyes need help working together comfortably. It is recorded in prism dioptres, often with a direction such as base in, base out, base up or base down.
Prism prescriptions should be ordered with care. They can be more complex to manufacture, particularly in stronger powers or certain frame shapes. If you are unsure how to enter prism values online, speak to an optician before placing your order rather than making an educated guess.
PD: your pupillary distance
Your PD, or pupillary distance, is the measurement in millimetres between the centres of your pupils. It helps position the optical centres of your lenses correctly in the frame.
PD is not always included on a glasses prescription because it is often measured during dispensing, once a frame has been selected. It can be a single number, such as 63 mm, or two monocular measurements, such as 31.5/31.5 mm. For varifocals, precise fitting measurements are especially important, including how the frame sits on your face.
How to read your glasses prescription correctly online
When ordering prescription glasses online, copy every figure exactly, including plus and minus signs. A missing sign changes the meaning of the prescription completely. Do not round numbers unless the ordering system instructs you to do so.
Check whether the form asks for CYL in plus or minus format. Many forms allow you to select the notation. If it does not match the format on your prescription, do not alter only the CYL figure or Axis yourself. A prescription conversion involves sphere, cylinder and axis together.
You should also select lenses that suit how you will use them. Everyday distance glasses, screen glasses, reading glasses and varifocals serve different purposes. The cheapest lens option is not always the best value if it does not match your work, driving, hobbies or preferred frame style. Lens thinning, anti-reflection coatings and photochromic options can make a noticeable difference, but whether they are worthwhile depends on your prescription and routine.
Prescription date and expiry: do not overlook it
A prescription is issued after an eye examination, but it is not a lifelong instruction. Your optometrist will recommend when you should return, based on your age, visual needs and eye health. For many adults this is every two years, though some people need more frequent checks.
If your glasses no longer feel right, you are experiencing headaches, eyestrain, double vision, sudden changes in sight, flashes or new floaters, do not rely on an old prescription. Arrange an eye examination promptly. Glasses can correct refractive error, but they do not replace professional assessment of your eye health.
When an expert check is worth having
Most straightforward single-vision prescriptions can be ordered easily when all the information is clear. More complex needs benefit from a conversation. This includes prism, high prescriptions, strong astigmatism, varifocals, occupational lenses, children’s glasses and any situation where a new frame is very different from your old one.
At an independent practice, the recommendation can start with your actual needs rather than a one-size-fits-all package. The Eye Centre can review your prescription, discuss lens choices and help ensure your frame and lenses work well together - whether you prefer to choose in practice or begin online.
Your prescription is there to make life sharper, more comfortable and more enjoyable. Keep a clear copy, enter it carefully, and ask for guidance whenever a figure or lens choice does not feel certain.
