How to Read Your Contact Lens Prescription

Table of Contents

• Why Your Contact Lens Prescription Is Different From Your Glasses Prescription
• What OD and OS Mean on Your Prescription
• How to Read Sphere (SPH), Power (PWR), and Plus or Minus Signs
• What Do BC and DIA Mean on Contacts?
• Cylinder (CYL) and Axis: Reading a Toric Contact Lens Prescription
• Contact Lens Prescription Example: A Field-by-Field Walkthrough
• How to Read Your Contact Lens Prescription When Fields Are Blank or Unclear
• Frequently Asked Questions

Last Updated: October 11, 2026

Why Your Contact Lens Prescription Is Different From Your Glasses Prescription

A contact lens prescription is not the same as a glasses prescription, and treating the two as interchangeable is one of the most common mistakes we see. Your glasses prescription measures how lenses sit about 12 millimeters from your eye. Your contact lens prescription measures how a lens sits directly on your cornea. (Source: National Eye Institute overview of refractive errors)

That difference changes everything. A contact prescription needs extra numbers your glasses script does not have, like base curve and diameter. It also has to account for how the lens sits and moves on your eye.

At Wilck Schwartz & Novak OD PC, we have fitted contact lenses for families across Brooklyn since 1964. One thing has stayed constant: you cannot order contacts from a glasses prescription, and a seller who lets you try is cutting corners.

What OD and OS Mean on Your Prescription

OD stands for oculus dexter, Latin for the right eye. OS stands for oculus sinister, meaning the left eye. Some prescriptions also list OU (oculus uter), which means both eyes together.

You will see OD listed first on almost every prescription. That is a convention, not a ranking. Each eye gets its own row of numbers because your two eyes rarely need the same correction.

Here is what to check first:

• OD = right eye
• OS = left eye
• OU = both eyes

If your two eyes have different values, that is normal. Most people are slightly uneven. What matters is that each eye's numbers get matched to the correct lens.

How to Read Sphere (SPH), Power (PWR), and Plus or Minus Signs

Sphere, often written as SPH or PWR, is the main power of your lens. It is measured in units called diopters, shown with a D. This number tells the lens how much to bend light so it lands correctly on your retina.

The sign in front of the number is the key:

• A minus sign (like -2.50) means nearsightedness, or myopia. You see close objects clearly but struggle with distance.
• A plus sign (like +1.75) means farsightedness, or hyperopia. Close work is harder, and distance may also blur.
• A number near zero means very little correction is needed.

The higher the number, the stronger the correction. A -6.00 is a much stronger prescription than a -1.00.

National Eye Institute overview of refractive errors

What Do BC and DIA Mean on Contacts?

BC stands for base curve. It describes the curve of your cornea and tells the lens how to sit on your eye. It is measured in millimeters, and most prescriptions fall between 8.0 and 9.5.

DIA stands for diameter. It measures the full width of the lens in millimeters. Typical soft lenses run from about 13.5 to 14.5.

These two numbers are why a contact lens prescription is different from a glasses prescription. Glasses do not touch your eye, so they do not need a base curve or diameter.

Get the BC wrong and the lens may slide, feel tight, or blur. That is a fitting problem, not a power problem.

Cylinder (CYL) and Axis: Reading a Toric Contact Lens Prescription

CYL stands for cylinder, and it corrects astigmatism. Astigmatism means your cornea is shaped more like a football than a basketball, so light focuses at two points instead of one.

Axis is a number from 1 to 180. It tells the lens where to sit at the right angle to correct that astigmatism. Think of it as a compass heading for the correction.

Lenses built for astigmatism are called toric lenses. They stay oriented on your eye, which is why the axis matters so much. If a toric lens rotates out of place, your vision blurs.

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A few things to know:

• CYL is usually a negative number, like -0.75 or -1.25
• Axis is always a whole number between 1 and 180
• If both fields are blank, you likely do not have astigmatism

Contact Lens Prescription Example: A Field-by-Field Walkthrough

Here is a sample contact lens prescription example so you can see how the fields work together, including a toric lens for one eye and a multifocal ADD value, which most abbreviation glossaries skip.

Reading it line by line:

• Right eye (OD): SPH -2.75 means moderate nearsightedness. No CYL or Axis, so no astigmatism correction is needed in this eye.
• Left eye (OS): SPH -3.00 is slightly stronger nearsighted correction, and CYL -0.75 with Axis 180 tells the lab this is a toric lens. The axis is the meridian the cylinder correction sits on, 180 is horizontal, 90 is vertical, and values in between are oblique. A toric lens has to stay rotated to that exact angle on your eye, which is why toric lenses use stabilization features like thin zones or prism ballast.
• ADD +2.00 on both eyes: This is the reading-power add for presbyopia. It is not a separate lens, it is built into a multifocal contact lens, which uses concentric zones or a segmented design to show distance, intermediate, and near at once. ADD is almost always the same number for both eyes even when SPH differs.
• BC 8.6 and DIA 14.2 on both eyes: Matching base curve and diameter is common but not guaranteed. If your two eyes measure differently, the prescription will list two different BC or DIA values, and you must order each eye separately.
• Brand and Replacement: A valid prescription can name a specific lens design. "Biweekly" is the replacement schedule, how often you discard and open a new pair, and it is separate from the prescription's expiration date.

Notice the two eyes are not identical. That is why each eye gets its own lens, and why you cannot split one eye's numbers across both.

FDA guidance on contact lenses as prescription medical devices

How to Read Your Contact Lens Prescription When Fields Are Blank or Unclear

A blank field is not automatically an error, but which field is blank changes what you should do next. Here is a practical decision path.

Blank CYL and Axis: You do not have astigmatism in that eye, so no toric correction is needed. This is normal and the prescription is complete.

Blank ADD: You are not being fit for a multifocal. If you are over 40 and struggling with close-up vision, ask your optometrist whether a multifocal or monovision fit is appropriate, do not assume the blank means you do not need one.

Blank BC or DIA: The prescription is incomplete. Base curve and diameter are fit parameters, not optional fields, and no legitimate seller should fill an order without them. Call the prescribing office and ask them to complete the record.

Blank brand or lens name: Some prescriptions specify a brand; others leave it open so any comparable lens in the same parameters can be dispensed. If yours is blank, ask whether substitution is allowed, some fits depend on a specific lens material or design.

Illegible or ambiguous handwriting: Do not guess. A misread minus sign, a 6 that looks like an 8, or a smudged axis value can produce a lens that is uncomfortable or unsafe. Ask the office to reissue a typed copy.

Conflicting values between eyes: If one eye lists CYL and the other does not, that is usually correct, astigmatism is often asymmetric. If the BC or DIA differ between eyes, that is also legitimate and each eye must be ordered separately.

Other fields worth confirming before you order:

• Wear schedule: Daily wear, extended wear, or daily disposable. Follow exactly what is written, extended wear has a higher infection risk and is not appropriate for every patient.
• Replacement schedule: How often you discard lenses (daily, biweekly, monthly). This is not the same as the prescription expiration date.
• Expiration date: A valid contact lens prescription has one. Ordering after it lapses is not safe, and most sellers will refuse the order.
• Prescriber and issue date: Required on a valid prescription. If either is missing, treat the document as incomplete.

If any field confuses you, call the office that wrote it. We would rather answer a question than have you guess. To schedule a fitting or renewal, book an appointment with our team and we will walk you through every number.