A contact lens prescription contains a series of abbreviations and numbers that describe your vision correction, the shape of your eye, and the specific lens that fits you. Once you know what each value means, reading the prescription takes about 30 seconds. Here’s a breakdown of every term you’ll find on a contact lens prescription or printed on the side of a lens box.
OD, OS, and OU: Which Eye Is Which
Every prescription lists values separately for each eye. OD refers to your right eye, OS refers to your left eye, and OU (less common) means both eyes share the same value. Your two eyes almost always have different correction needs, so expect the numbers on the OD and OS lines to differ.
Power (PWR) or Sphere (SPH)
This is the most important number on your prescription. It tells you how much correction your lenses provide, measured in units called diopters. A minus sign means you’re nearsighted (you see nearby objects clearly but distant ones are blurry). A plus sign means you’re farsighted (distant objects are clearer than close ones).
The farther the number is from zero, the stronger the correction. So a prescription of -4.50 is a stronger correction than -1.75. You’ll see this labeled as PWR, SPH, or sometimes just “Power” depending on the brand.
Base Curve (BC)
The base curve describes how curved the back surface of the lens is, measured in millimeters. Most soft contact lenses fall between 8.0 and 9.5 mm. A lower number means a steeper curve, which fits a more sharply curved cornea. A higher number means a flatter curve. Your eye care provider measures this during your fitting to make sure the lens sits comfortably and stays centered on your eye. Getting the wrong base curve can cause the lens to feel tight, slide around, or irritate your cornea.
Diameter (DIA)
Diameter is the total width of the lens from edge to edge, also in millimeters. Soft lenses typically range from 13.0 to 14.5 mm. This value ensures the lens covers your cornea properly. Like the base curve, the diameter is determined during a fitting and matched to your eye’s dimensions. You won’t usually have a choice in this number.
Brand Name
Unlike glasses prescriptions, contact lens prescriptions include a specific brand. This matters because each brand uses different materials that affect how much oxygen reaches your cornea, how the lens holds moisture, and how it sits on your eye. You can’t swap one brand for another without a new fitting, even if the power is the same. The one exception is private label lenses: if your prescription lists a store-brand lens, the seller can substitute the identical manufacturer-brand lens it’s based on.
CYL and Axis: Astigmatism Correction
If you have astigmatism, your prescription will include two additional values. These only appear on prescriptions for toric lenses, which are specially designed to correct the uneven curvature that causes astigmatism.
Cylinder (CYL) measures the degree of astigmatism and is written with a minus sign, in diopters. A higher number means more astigmatism correction. Axis (AX) is a number between 0 and 180 that describes the angle, in degrees, where the cylinder correction needs to be positioned on the lens. Think of it as a compass heading that tells the lens which direction to aim its astigmatism correction. Both values work together: the cylinder says how much extra correction you need, and the axis says where to put it.
ADD Power: Multifocal Lenses
If you wear multifocal or bifocal contact lenses, your prescription includes an ADD value. This represents extra magnifying power built into part of the lens to help with reading and close-up work, a common need for people over 40 with presbyopia.
The ADD value may appear as a specific number in diopters (like +1.50) or as a general label: Low, Medium, or High. Low add powers tend to produce the least interference with distance vision. Higher adds provide more reading help but can introduce some blur when looking at things far away. Your provider may also prescribe different add levels for each eye, placing the lower add on your dominant eye to keep distance vision sharper.
Why Your Contact Prescription Differs From Glasses
If you wear both contacts and glasses, you’ve probably noticed the power numbers don’t match. This isn’t a mistake. Glasses sit about 12 millimeters in front of your eye, while contacts rest directly on the surface. That gap changes how the lens bends light, so the power needs to be recalculated. For mild prescriptions the difference is small, but for stronger corrections it becomes significant. A contact lens prescription also includes fitting data (base curve, diameter, brand) that a glasses prescription never needs, since glasses don’t touch your eye at all.
Reading the Box
When you receive your contact lenses, every value from your prescription is printed on the box and on the foil of each individual lens blister pack. The abbreviations are the same ones listed above: PWR or SPH, BC, DIA, and (if applicable) CYL, AX, and ADD. Before you open a new box, it’s worth checking these numbers against your prescription to make sure you received the right lenses, especially if you order different corrections for each eye.
One value on the box that isn’t part of your prescription is the expiration date of the lenses themselves. This is separate from the expiration date of your prescription.
Prescription Expiration
Contact lens prescriptions don’t last forever. Under federal rules enforced by the FTC, a prescription is valid for at least one year from its issue date. Some states set longer expiration periods (commonly two years), and if your state allows a longer window, that longer period applies. Your provider can set a shorter expiration only if there’s a documented medical reason. Once a prescription expires, you’ll need a new eye exam and fitting before you can reorder lenses.