How to read your contact lens prescription

Shopping for contact lenses online? Great! You’re about to save yourself valuable time and money. But first, you need to know which lenses are right for you and how to read that prescription from your optician. As you get started with lenses and 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, bi-weekly lenses 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, your prescription for spectacles is NOT the same as a prescripion for contacts, but more on that later. Now, let's break down all those numbers and abbreviations on your prescription.

Overview of Parameters

Your prescription has three basic components: your parameters, recommended brand(s) of lenses, and any special instructions . To break it down for you, we have created a quick overview of all the parameters and their common abbreviations.

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 -)
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
BC Base Curve indicates the degree of curvature and how the lens will fit to the eyeball
DIA Diameter describes the width of the lens
PL Plano indicates colour lenses without power
EXP Expiration date indicates when the lenses will expire

This table will guide you through selecting the correct parameters according to your optician’s advice . Although it provides many answers, you may still have questions. Below, we’ve addressed some of our customers’ most frequently asked questions.

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). 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 . If you have the same prescription for both eyes, there’s no need to select the second attributes! Just fill in your parameters once, and select the number of boxes you require.

select your attributes for your second eye on Alensa

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. If you have accidentally ordered the wrong value, you can always return unopened boxes according to our Exchange & Return Policy.

Can I use my glasses prescription for contacts?

NO. The parameters for glasses and contacts can be very different , due to where the lenses of each sit in relation to the eyeball. 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.

How do I choose cylinder and axis for toric lenses?

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 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. CooperVision lenses have a further distinction between the dominant and non-dominant eye, so if you have any questions about your ADD power for each eye, consult with your optician.

Are base curve and diameter 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 millimeters, 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?

At, 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

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 Irish law, it is the buyer's responsibility to maintain an up-to-date prescription before purchasing lenses online.

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 # on our TopVue lenses. If, for any reason you find them unsuitable, you may return them.

Prescription Verification - only applies to the USA and Korea

We strive to provide the best care possible to our customers. As such, we have implemented a verification system that is in compliance with the FCLCA and Rules for USA and Korea customers. Under the Fairness to Contact Lens Consumer Law (FCLCA), we are required to verify your contact lens prescription.

We provide two easy methods of verifying your contact lens prescription during your checkout process. The most efficient way is to upload your prescription slip to your Alensa account.

You can provide us a copy of your valid prescription written by eye care professionals. You can either scan or snap a picture of your written prescription and send it to us: . Once we get your prescription, we will keep the record for you, you can check out anytime without the need to re-upload again. Providing invalid or inaccurate prescription may result in order cancellation.


Do you have any questions?