Eye Health

Understanding Your Contact Lens Prescription: OD, OS, BC, DIA Explained

Your contact lens prescription looks intimidating the first time you see it. Numbers, letters, plus and minus signs — what does any of it mean? Once you understand the abbreviations, ordering the right lenses becomes straightforward.

The Quick Reference Table

Abbreviation	Full Name	What It Means	Example Value
OD	Oculus Dexter	Your right eye	—
OS	Oculus Sinister	Your left eye	—
OU	Oculus Uterque	Both eyes (same Rx)	—
SPH / PWR	Sphere / Power	Correction strength for nearsight/farsight	-2.50, +1.75
CYL	Cylinder	Astigmatism correction (only in toric lenses)	-0.75, -1.25
AXIS	Axis	Orientation of astigmatism correction (0–180°)	010, 090, 180
BC	Base Curve	Curvature of the lens (must match your eye)	8.4, 8.6, 8.8
DIA	Diameter	Width of the lens in millimeters	13.8, 14.0, 14.2
ADD	Addition	Extra magnification for multifocal lenses	+1.50, +2.00
Brand	—	Specific lens your doctor fitted you for	ACUVUE OASYS

Sphere (SPH) — Your Core Vision Correction

This is the main number. A negative SPH (−) means you're nearsighted (myopia): you see nearby things clearly but distance is blurry. A positive SPH (+) means you're farsighted (hyperopia).

The further from zero, the stronger the prescription. −1.00 is mild; −6.00 is quite strong. Most people fall between −0.50 and −5.00.

Cylinder and Axis — Only If You Have Astigmatism

If your prescription has CYL and AXIS values, you have astigmatism — your eye is shaped slightly like a football rather than a basketball. You'll need toric contact lenses (not standard spherical lenses).

The AXIS tells the toric lens which direction to rotate so the cylinder correction aligns with your astigmatism. This is why toric lenses have a tiny weighted bottom — to keep them properly oriented.

Base Curve (BC) — Fit, Not Vision

BC is measured in millimeters (typically 8.3–9.0). It describes the curvature of the back of the lens. If BC is too steep or too flat for your eye, the lens won't sit properly and your vision will fluctuate. Your eye doctor measures your BC during a contact lens fitting — never guess this number.

Diameter (DIA) — Size of the Lens

DIA is the total width of the lens. Most soft lenses are 13.8–14.5 mm. Like BC, DIA is determined by your fitting exam. Different brands use different diameters even for the same correction strength.

Why You Can't Use Your Glasses Prescription for Contacts

Glasses sit 12mm from your eye; contacts sit directly on it. This changes the effective power needed. Also, glasses prescriptions don't include BC or DIA, and the cylinder values are measured differently. You need a separate contact lens exam and fitting before ordering contacts.

Reading a Sample Prescription

Here's what a full toric prescription looks like:

Eye	SPH	CYL	AXIS	BC	DIA	Brand
OD (Right)	-2.50	-0.75	010	8.6	14.4	Biofinity Toric
OS (Left)	-3.00	-1.25	180	8.6	14.4	Biofinity Toric

This person is nearsighted in both eyes, has astigmatism, and requires toric lenses. Their right and left eyes have different SPH and CYL strengths — completely normal.