How to read your prescription for glasses or lenses

So, you’ve gotten your hand on a prescription, and you're looking to order contact lenses and glasses online? It might not be easy interpreting what’s written there, so we’ve put together a small guide to help you. We’ve also included some common questions regarding prescriptions for glasses or lenses. Take a look below and don’t hesitate to contact us in case you have any further questions.

General overview of parameters and questions

On your prescription, you’ll see all kinds of parameters and values. To break it down for you, we have created a quick overview of all the parameters and their common abbreviations for both glasses and lenses.

Please remember that you should renew your prescription at least once a year. Even if you haven't noticed any changes in your vision, it's important to be examined by your optician. Under UK law, it is the buyer's responsibility to maintain an up-to-date prescription before purchasing online.

Abbreviation Explanation
OD / R Oculus Dexter indicates the parameters for the Right eye
OS / L Oculus Sinister indicates the parameters for the Left eye
OU Oculus Uterque indicates when parameters are identical for both eyes
SP(H), PWR or D Sphere, Power, or Dioptre value indicates the value of near or farsightedness (+ or -). A possible (=) sign indicates (-).
CYL or CP Cylinder indicates astigmatism and tells the vertical curve of your eyeball
X, AX, AXE Axis indicates the degree and direction of the cylinder
ADD ADD power indicates the extra strength for multifocal lenses
D and N For your ADD power, these letters simply differentiate between you dominant (D) and non-dominant (N) eye
BC Base Curve is used specifically for lenses and indicates the degree of curvature and how the lens will fit on the eyeball
DIA Diameter is used specifically for lenses and describes the width of the lens
PL Plano indicates coloured contact lenses without power
EXP Expiration date indicates when the lenses will expire
PD Pupillary distance is a value used specifically for glasses and indicates the distance between your pupils. The exact distance is important in order for us to correctly centre your lenses on your eyes.

Plus or Minus?

Are you shortsighted (myopic), meaning you can’t see well in the distance? Then your power will be minus. Farsighted, or struggling to see close objects? This condition is also called hyperopia, and it means your power will be plus. This value can be indicated on your prescription as either “D” diopter value; “PWR” power; or “SPH” sphere. Be careful when selecting your power, as a small slip-up can cause a big difference in eyesight. For lenses, if you have accidentally ordered the wrong value, you can always return unopened boxes according to our Exchange & Return Policy.

How do I choose cylinder and axis?

Cylinder and axis are two measurements in toric lenses necessary to correct astigmatism. The cylinder is an extra correction that denotes the severity of astigmatism and has a minus value expressed in increments of 0.25. The axis is expressed in degrees between 0° and 180°. We advise you to consult with your optician if you have any doubts regarding these two parameters.

What does the ADD power mean?

If you suffer from presbyopia and require multifocal contacts, your prescription may include an ADD, or additional power to help with near vision. ADD powers range from +0.50 to +3.00. Some brands of lenses offer ADD power in low, medium or high values. “D” and “N” may further indicate which eye is the dominant eye (D) and which is non-dominant (N). Some manufacturers of lenses require you to properly select this as well. In case you have any doubts about this, consult us or your optician first.

Right or Left Eye?

Your optician may indicate this with a simple “L” and “R” or the Latin terms “Oculus Dexter” (right) and “Oculus Sinister” (left). When ordering glasses, simply input the correct values for your left and right eye. In case you’re ordering lenses, if you require a different power for each eye, you’ll need to click “Select attributes for second” eye when placing your order on our website. When placing your order, there is no distinction between left and right, so just think of them as Eye #1 and Eye #2 (be sure to make sure whichever values input correspond to the correct eye). If you have the same prescription for both eyes, there’s no need to select the second attribute! Just fill in your parameters once, and select the number of boxes you require.

select your attributes for your second eye on Alensa

What is the difference between D and N in contact lenses?

If you use multifocal or bifocal eyeglasses, it is necessary to determine your dominant and non-dominant eye for the correct vision correction. The dominant eye, commonly indicated as "D" focuses primarily on distant vision, whereas the non-dominant eye, indicated as "N" is associated with near vision. We strongly recommend consulting with your eye care specialist in case you need lenses with this addition.

Can I use my glasses prescription for contacts?

NO. Due to where the lenses of each sit in relation to your eyeball, the parameters for glasses and contacts can be very different. Glasses sit a few centimeters in front of your eyes, while contact lenses fit directly on top of the eyeballs. Your glasses prescription does not contain the necessary information about the appropriate contact lens base curve and diameter that’s right for you.

Tips for understanding and using your prescription specifically for glasses

A prescription for glasses is a lot simpler to interpret once it's clear what each value represents (see table above). You won’t need to worry about values like BC and DIA, which are only relevant for contact lenses. There is, however, one important value you’ll need in order to purchase glasses online, which is the pupillary distance (PD). This value allows us to properly centre your eyeglass lenses. Some eye care specialists don’t mention it at all, in which case you’ll need to ask them about it. Some eye- specialists mention only one PD value when measuring. This means that both your pupils are at an equal distance from the bridge of your nose. If that's the case, you will need to divide the total value by 2 to get the right and left eye's values. For most people, the distance from the pupil to the bridge of the nose is between 28-35 millimetres; however, there are exceptions. If your doctor or eye-specialist has not added this to your prescription, you can measure it yourself with our PD measurement tool .

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Tips for understanding and using your prescription specifically for lenses

After you have had an eye exam, your optometrist will suggest a particular brand and model of contact lenses for you based on the characteristics of your eyes and the nature of your vision problems. Not only is there a difference between daily disposables and bi-weekly and monthly contact lenses, but also different brands will likely feel different from person to person, so it’s important to follow your optometrist’s suggestions. Don’t forget you also need an eye exam and prescription before ordering coloured contact lenses, even those without vision correction. And, the all-important rule: your prescription for spectacles is NOT the same as a prescription for contact lenses. Not only will the numbers be different for both prescription, but a lens prescription will also include a value for base curve (BC) and diameter (D).

Base curve and diameter are important when choosing contact lenses

The base curve refers to the degree of curvature of the contact lens, or how closely it fits against the eyeball. A lower BC, like 8.40, means the lens is more curved and will fit snugly against the eye. Higher BCs like 8.70 indicate a flatter lens. The diameter refers to the width of the lens and is expressed in millimetres, usually between 13 and 15 mm. A slight difference (0.1 or 0.2 mm) will likely not have any noticeable impact on your comfort or quality of vision. But wearing lenses with a significantly different BC or DIA than you were prescribed can sometimes lead to discomfort and distorted vision, so it’s critical to follow the recommendation on your prescription.

I’ve lost my paper prescription. Where can I find this information?

With Alensa, you may buy contacts without a prescription. If it’s been less than one year since your last eye exam, and you’ve not experienced any changes in vision quality, you can always find the necessary prescription information on your box of lenses. We’ve decoded it for you here:

Find out how you can decode the parameters on your contact lenses box

Can I order trial lenses?

Please note that we are not authorised to provide trial lenses. Only your eye-care specialist knows your eyes inside and out and can prescribe lenses on a trial basis. Although they’re not considered trial lenses, we do offer a Money-back guarantee on our TopVue lenses. If for any reason you find them unsuitable, you may return them.

Comments (3)

Do you have any questions
Claire MacLachlan

Hi Alensa Customer Support Team, I am currently using clarities 1 day contact lenses under the supervision with Optical Express my prescription is Left Eye +2.00 Right Eye 6.50 both DIA 14.1 BASE 8.6 I see very clearly close up and at distance but have difficulty seeing faces and coloured writing at mid distance. I have read up on multi focals lenses and would like to give Pro Clear 1 day multifocals a try, would I need to purchase for both eyes or just my dominant left eye, Many Thanks in Advance


Hi, Claire, If both your eyes have different prescriptions, you'll need lenses for both eyes.

Thanks, Alensa


Hi I would like to try out your contact lenses


Hi, Kenneth, If you could send your prescription to our customer service at, we'd be more than happy to help you!

Thanks, Alensa


I have multi focal daily contacts, I would like monthly. My BC is 8.4 and my DIA14.3 what lens would be similar for me to order as I can't find any the same. I am using acuvue moist dailies. Thank you