Reading a contact lens prescription is easier than it looks — SPH is your lens power, BC is the base curve (how curved the lens is), DIA is the diameter (how wide it is), and OD/OS simply tell you which eye each line belongs to. In this guide, we break down every abbreviation you’ll see, walk through a real prescription line by line, and show you exactly what to enter when you order colored contacts from us.
In this guide:
What OD, OS, and OU mean (right eye, left eye, both)
SPH, BC, and DIA — and what each number tells you
CYL, axis, and add — only if your prescription includes them
Expiration dates, and what to enter at our checkout
Frequently Asked Questions
What does OD and OS mean on a contact lens prescription?
OD stands for oculus dexter and means your right eye; OS stands for oculus sinister and means your left eye. OU means both eyes. Your two eyes often have different powers, so always match each eye’s numbers to the correct box when ordering.
Is a contact lens prescription the same as a glasses prescription?
No. A glasses prescription only lists lens power, while a contact lens prescription also includes the base curve (BC) and diameter (DIA) of the lens, which are measured during a contact lens fitting. You cannot order contacts using a glasses prescription alone.
What does BC mean on a contact lens prescription?
BC stands for base curve — the curvature of the inside of the lens, measured in millimeters. It should match your cornea’s curve so the lens fits comfortably. Typical values are 8.4–8.8 mm, with 8.6 being the most common.
What does DIA mean on a contact lens prescription?
DIA stands for diameter — the width of the lens in millimeters. Most lenses measure 14.0–14.5 mm. A larger diameter covers more of the iris, which gives colored contacts a bolder, more visible color change.
How long is a contact lens prescription valid?
Most contact lens prescriptions expire after one to two years, depending on your state. Always check the expiration date before ordering — an expired prescription cannot be used, and you’ll need a quick new exam to renew it.
Can I order colored contacts with my glasses prescription?
No — U.S. regulations require a valid prescription for all contact lenses, including zero-power cosmetic lenses, and a contact lens prescription is different from a glasses prescription. Book a contact lens exam so your doctor can fit the lens and write the right prescription.
