How to read your contact lens prescription: OD vs. OS

Your contact lens prescription is a string of abbreviations and numbers that mean nothing until someone explains them.  

So, let's do that. 

Here's what every field means, how to match each number to the right (as in correct) eye, and what to double-check before you order.  

This is the first thing to get right, because mixing these up means ordering the completely wrong prescription for the wrong eye. 

• OD (Oculus Dexter) = right eye 

• OS (Oculus Sinister) = left eye 

• OU (Oculus Uterque) = both eyes 

These come from Latin, which is medicine's way of keeping things unnecessarily complicated. The practical translation: OD is right, OS is left. On most prescriptions, OD is listed first, then OS. 

OU shows up less often on contact lens prescriptions. When it does, it applies the same value to both eyes, usually for something like add power in multifocal lenses. If your eyes need different corrections, you'll still see separate OD and OS lines. 

Here's a sample prescription line to walk through: 

OD: SPH -2.50 / BC 8.6 / DIA 14.2 / CYL -0.75 / AXIS 180 

SPH, sometimes labeled as "power" or "PWR," is the main vision correction number. It's measured in diopters and written with a plus or minus sign: 

• Minus (-) = nearsightedness (myopia). You see up close fine but struggle at a distance. 

• Plus (+) = farsightedness (hyperopia). Distance vision may be clearer than near vision. 

Higher numbers mean stronger correction, whether positive or negative. A -4.00 is a stronger prescription than a -1.50. A +3.00 is stronger than a +1.00.  

In the case of myopia, the higher the number, the shorter the visual range. If someone has –0.25, their myopia is mild, and they probably only need glasses when driving. The SPH in our example, -2.50, can most likely see their phone screen without any correction, but will need their glasses daily. 

These two always appear together. CYL (cylinder) and axis only appear on your prescription if you have astigmatism, a condition where the cornea is slightly irregular in shape rather than perfectly round. 

• CYL shows how much astigmatism correction you need, also in diopters 

• Axis shows the orientation of that correction, written in degrees from 1 to 180 

If these figures are on your prescription, you'll need toric contact lenses, which are specifically designed to stay correctly aligned on an eye with astigmatism. 

Base curve is a measurement of the curvature of the back of the lens, the side that sits on your cornea. It's written as a number like 8.4, 8.6, or 8.8. 

This isn't a vision correction number. It's a fit number. A lens with the wrong base curve can feel uncomfortable, move around too much, or sit too tightly on the eye. Base curve is specific to contact lenses and won't appear on a glasses prescription. 

Diameter is the width of the lens from edge to edge, measured in millimeters. Most soft contact lenses fall between 13.8 and 14.5mm. 

Like BC, diameter is about fit, not vision strength. It's also contact-lens-specific, so you won't find it on a glasses prescription. 

If you're over 40 and starting to struggle with close-up reading, your prescription may include an ADD value. This is an additional correction for near vision that gets built into multifocal or bifocal contact lenses. 

ADD is written as a positive number (like +1.50 or +2.00) and represents the extra magnification needed for reading. Not everyone has or needs an ADD value on their prescription.  

This is one of the most common points of confusion, and it matters a lot when you're trying to order. 

Glasses sit a few millimeters in front of your eye. Contact lenses sit directly on your cornea. That distance changes how the lens needs to correct your vision, especially at higher prescriptions. A glasses prescription and a contact lens prescription for the same person may have different SPH values as a result. 

If your prescription doesn't include base curve and diameter, it's probably a glasses prescription. You'll need a contact lens exam and fitting to get a complete contact lens prescription. 

When you enter your prescription during checkout, you'll fill in separate fields for OD (right eye) and OS (left eye). Double-check each value against the correct line on your prescription before submitting. 

Common mistakes: 

• Entering OD values in the OS field and vice versa 

• Skipping CYL or axis when you have astigmatism 

• Assuming both eyes are the same prescription 

A few seconds of double-checking saves a lot of hassle. 

Before you order, run through this quick checklist: 

• Is this a contact lens prescription or a glasses prescription? Check for base curve and diameter. If they're missing, it may be glasses-only. 

• Are OD and OS in the correct spots? Right eye first, left eye second. Confirm before entering values. 

• Does your prescription include CYL and axis? If you have astigmatism and you're leaving those fields blank, you'll get the wrong lenses. 

• Is the prescription current? Check the expiration date. An expired prescription may block your order during verification. 

• Does the lens brand match what you're ordering? Contact lens prescriptions are often brand-specific. Switching brands usually requires a new fitting. 

Reach out to your eye doctor if: 

• Any field on your prescription is hard to read or seems incomplete 

• You're unsure whether what you have is a contact lens or glasses prescription 

• Your prescription is close to expiring or has already expired 

• You're switching to a new lens brand or type 

Contact lens prescriptions are valid for at least one year under federal law, though some states allow longer periods. Check the expiration date on your prescription and confirm with your doctor if you're unsure.  

No. A glasses prescription doesn't include base curve, diameter, or the brand-specific fitting information required for contact lenses. You need a separate contact lens prescription from a contact lens exam. 

Because base curve, diameter, and material vary between brands, contact lens prescriptions are often specific to a particular lens. Switching brands typically requires a new fitting with your eye doctor to confirm the new lens fits correctly. 

Skip the extra trips and get what you need, when you need it. With 1-800 Contacts, you can order your lenses in just a few clicks and have them delivered right to your door. Need to renew your prescription? Our online vision exam makes it easy to do from home. New customers save on their first order.