A contact lens box may look simple, but the numbers printed on it are specific to your eyes. Choosing the wrong power, base curve, or lens type can affect clarity and comfort. This contact lens prescription guide helps you read the details with confidence, so replacing your regular lenses online feels straightforward.
Your contact lens prescription is not just a shopping reference. It is a clinical fitting recommendation based on your vision, eye shape, lens material, and how your eyes respond to wear. When it is time to reorder, match every field exactly with your current valid prescription or your existing lens box.
Why a Contact Lens Prescription Is Different
A glasses prescription and a contact lens prescription are related, but they are not interchangeable. Glasses sit a short distance in front of your eyes, while contact lenses rest directly on the surface of the eye. That difference can change the lens power required, particularly with stronger prescriptions.
A contact lens prescription also includes fit details that glasses do not need. The base curve and diameter help determine how a lens sits and moves on your eye. Even if the power looks right, switching to a different fit or material without professional advice may lead to dryness, blurred vision, or an uncomfortable lens that does not stay centred.
For that reason, order the brand, lens type, and parameters your eye care professional prescribed. If you are considering a change – perhaps from monthly lenses to daily disposables, or from a standard spherical lens to one designed for astigmatism – book an eye exam and fitting first.
How to Read Your Contact Lens Prescription
Most prescriptions use abbreviations. They may appear on a printed prescription, a confirmation from your clinic, or the side of your current lens packaging. Here is what the common terms mean.
OD and OS
OD means oculus dexter, or right eye. OS means oculus sinister, or left eye. Some prescriptions also use R and L instead. Do not assume both eyes use the same parameters. Many people have a different power in each eye, and some may wear a corrective lens in only one eye.
SPH or PWR
SPH, sometimes shown as PWR, means sphere or power. This is the correction for nearsightedness or farsightedness.
A minus sign, such as -2.50, generally corrects nearsightedness. A plus sign, such as +1.75, generally corrects farsightedness. The sign matters. Entering +2.50 when your prescription is -2.50 is not a small error – it is a completely different correction.
BC: Base Curve
BC stands for base curve. It describes the curve of the back surface of the lens, usually shown in millimetres. Common values include 8.4, 8.5, 8.6, and 8.7, but the correct value depends on the lens design and your eye.
It can be tempting to think a close number will work just as well. In practice, the base curve is tied to the prescribed lens model. A lens with an 8.6 BC from one brand is not automatically a substitute for a lens with an 8.6 BC from another.
DIA: Diameter
DIA means diameter, or the width of the lens in millimetres. Soft contact lenses often have a diameter around 14.0 to 14.5 mm, although the range varies by lens type.
Like the base curve, diameter contributes to the overall fit. Match it exactly when ordering the same lens. If your prescription lists a diameter that is unavailable in a product you are considering, that product is likely not your prescribed replacement.
CYL and AXIS for Astigmatism
If you have astigmatism, your prescription may include CYL and AXIS. CYL, short for cylinder, indicates the amount of astigmatism correction. It is shown as a positive or negative number, depending on the prescription format.
AXIS identifies the direction of that correction on a scale from 1 to 180 degrees. It is not a power measurement, so 090 and 180 are not interchangeable. Toric contact lenses are designed to settle in a particular position on the eye, allowing the cylinder and axis correction to stay aligned for clearer vision.
Toric lenses may also include a manufacturer-specific stabilization design. This is one reason it is best to stay with your prescribed brand unless your eye care professional recommends a different option.
ADD for Multifocal Lenses
ADD, or addition, appears on multifocal contact lens prescriptions. It provides extra focusing support for close-up tasks such as reading a phone, checking labels, or working at a screen.
Depending on the brand, ADD may be written as a number or grouped as Low, Medium, or High. Use the exact add power or category shown on your prescription. A multifocal lens is designed to balance distance, intermediate, and near vision, so changing this value can affect how naturally your vision feels throughout the day.
Your Contact Lens Prescription Guide to Lens Types
The numbers tell part of the story. The lens replacement schedule and design matter just as much.
Daily disposable lenses are worn once and discarded at the end of the day. They can be a convenient choice for busy schedules, travel, sport, and people who prefer not to clean and store lenses. Monthly or biweekly lenses are replaced on a schedule and require proper cleaning, disinfecting, and storage every time they are removed.
Spherical lenses correct standard nearsightedness or farsightedness. Toric lenses are made for astigmatism, while multifocal lenses support vision at more than one distance. Some lenses also have features intended to support moisture retention, reduce the feeling of dryness, or filter certain UV rays. These benefits can be useful, but they do not replace sunglasses and do not make a lens suitable for everyone.
Comfort is personal. A lens that works well for a friend may not be the best fit for your eyes, workday, screen use, or wearing schedule. The right choice is the one that matches your professional fitting and gives you consistently clear, comfortable vision.
Before You Place an Online Order
Take a moment to compare your order with a current prescription or an unopened box from your latest supply. Check the right and left eye separately, then confirm the lens brand, power, BC, DIA, and any CYL, AXIS, or ADD values.
Also confirm the replacement schedule and the number of lenses per box. A 30-lens daily disposable box generally covers one eye for 30 days, while a six-lens monthly box can last one eye for six months when worn as directed. Packaging quantities vary, so this small check helps you order the supply that fits your routine.
Keep an eye on your prescription expiry date as well. Regular eye exams do more than update your numbers. They allow your eye care professional to check eye health, lens fit, dryness concerns, and whether your current lenses still suit your needs.
When Not to Reorder the Same Lenses
A reorder is easy when your vision is clear and your current lenses remain comfortable. It is worth booking an appointment instead if you notice persistent redness, stinging, dryness, fluctuating vision, headaches, light sensitivity, or a lens that suddenly feels wrong.
Do not try to solve ongoing discomfort by wearing lenses longer, sleeping in lenses not approved for overnight wear, or switching brands based only on price. Dry eye products may support comfort for some wearers, but recurring symptoms deserve professional guidance. Contact lens comfort should not be something you simply tolerate.
If you are replacing a familiar prescription, Precisely Contacts makes it easier to select the parameters you already know and order with confidence from Canada. Keep your prescription details handy, choose your prescribed lens, and give your eyes the consistent care they deserve.
