Every contact lens blister pack has a small foil lid printed with abbreviations and symbols that identify exactly what’s inside. Once you know what each one means, you can quickly confirm you’re putting the right lens in the right eye and that it hasn’t expired. Here’s a breakdown of everything printed on that foil.
Power (PWR or SPH)
This is the number most people recognize. PWR or SPH tells you the corrective strength of the lens, measured in diopters. A minus sign (−) means the lens corrects nearsightedness, and a plus sign (+) means it corrects farsightedness. The higher the number after the sign, the stronger the correction. So −3.00 is a stronger nearsighted prescription than −1.00.
If your two eyes have different prescriptions, this is the value you’ll use to tell your boxes apart. Check the power on the blister pack against the corresponding line on your prescription to make sure you’re opening the right one.
Base Curve (BC)
BC is the curvature of the lens, measured in millimeters. For soft contact lenses, this number typically falls between 8.0 mm and 9.0 mm. A lower number means a steeper curve, while a higher number means a flatter one. Your eye care provider selects a base curve that matches the shape of your cornea so the lens sits comfortably and stays centered. You don’t choose this value yourself, but you should confirm it matches your prescription when you open a new pack.
Diameter (DIA)
DIA is the full width of the lens from edge to edge, also in millimeters. Most soft lenses have a diameter between 13 mm and 15 mm. Like base curve, this is fitted to your eye and shouldn’t vary from what your prescription specifies. It’s usually the same for both eyes, but not always.
Extra Values for Astigmatism: CYL and AX
If you wear toric lenses (lenses that correct astigmatism), your blister pack will have two additional numbers. CYL, short for cylinder, indicates the amount of astigmatism correction the lens provides. It works like the power value but specifically compensates for the irregular shape of your cornea. The higher the CYL number, the more correction you need.
AX, or axis, is a number between 0 and 180 that describes the angle of your astigmatism. Think of it as the direction the correction is oriented. If your cornea is shaped more like a football than a basketball, the axis tells the lens which way that football is tilted. Both CYL and axis are unique to each eye, so check them carefully when your left and right prescriptions differ.
ADD Power for Multifocal Lenses
Multifocal contact lenses include an ADD value, which is the extra magnifying power built into the lens for reading and close-up tasks. Some brands list this as a specific diopter number (like +1.50), while others use categories such as Low or High. Bausch + Lomb, for example, defines Low as +0.75 to +1.50 diopters and High as +1.75 to +2.50 diopters. Your prescription will specify which ADD level you need.
OD and OS: Which Eye Is Which
OD stands for the right eye and OS stands for the left eye (from the Latin “oculus dexter” and “oculus sinister”). These labels sometimes appear on the outer box but not always on individual blister packs. Many blister packs only show the lens parameters, so if your pack doesn’t say OD or OS, match the power, base curve, and any CYL/axis values to the corresponding line on your prescription to figure out which eye the lens belongs to. If both eyes share identical parameters, it doesn’t matter which pack you pull from.
Lot Number
The lot number (often preceded by the word “LOT” or a small factory symbol) identifies the specific manufacturing batch. You won’t need this day to day, but it becomes important if there’s ever a product recall. Keeping your boxes until you’ve used all the lenses inside makes it easy to check lot numbers against any safety notices from the manufacturer.
Expiration Date
The expiration date appears as a year and month, sometimes next to a small hourglass symbol or the abbreviation “EXP.” This is the date by which the lens should be used. An unopened blister pack keeps the lens sterile in its sealed saline solution, but that sterility isn’t guaranteed past the printed date. If a pack has expired, don’t use it, even if the foil seal looks intact.
Sterilization and Other Symbols
You’ll often see small icons on the foil that aren’t accompanied by words. A common one indicates how the lens was sterilized, whether by steam, irradiation, or another method. These are standardized medical device symbols. Manufacturers are required to include a symbols glossary somewhere in the packaging (usually a small printed insert or on the box itself) that explains each icon.
Some blister packs also note that the lens has a visibility tint, meaning it’s lightly colored (usually blue or green) so you can see it more easily during handling. This tint doesn’t change your eye color or affect your vision. Packs may also indicate UV blocking if the lens material absorbs ultraviolet light, though this is noted as a feature, not a substitute for sunglasses.
Putting It All Together
A typical blister pack foil for a standard soft lens might read something like:
- PWR: −2.50
- BC: 8.4
- DIA: 14.0
- LOT: A12345
- EXP: 2026-09
A toric lens adds CYL (for example, −1.25) and AX (for example, 180). A multifocal lens adds an ADD value (for example, Low or +2.00). Every one of these values should match a line on your prescription. If anything looks off, compare the blister pack to your prescription printout before putting the lens in your eye. Mismatched base curves or cylinder values can cause discomfort, blurry vision, or lenses that won’t stay in place.