A contact lens prescription has one job your glasses prescription cannot do: it describes a lens that sits on your eye rather than 12 mm in front of it. That is why it carries a base curve, a diameter and a brand name, and why the powers are not the same numbers.
Every field, decoded
| Field | What it means |
|---|---|
| OD | Oculus dexter — right eye |
| OS | Oculus sinister — left eye |
| OU | Both eyes |
| PWR / SPH / D | Power, in dioptres. Minus is short-sighted, plus is long-sighted. |
| BC | Base curve — the curvature of the back of the lens, in mm |
| DIA | Diameter — how wide the lens is, in mm |
| CYL | Cylinder — the amount of astigmatism. Only on toric prescriptions. |
| AXIS | The orientation of that astigmatism, 0–180 degrees |
| ADD | Extra near power for presbyopia. Often LOW / MED / HIGH. |
| Brand | The specific product. Legally part of the prescription. |
| Expiration | Usually one year from issue; longer in some states |
Base curve: the number people ignore
Base curve is the radius of the lens’s back surface, typically 8.3 to 9.0 mm. Lower numbers are steeper; higher numbers are flatter.
It has to correspond to the shape of your cornea, and the window is narrow:
- Too flat and the lens slides around, moves excessively on blinking, and feels like an edge on your eye
- Too steep and it grips, restricting the tear exchange underneath — which is the one that does not announce itself. A tight lens can feel perfectly comfortable while restricting oxygen and debris clearance.
Because manufacturers use different geometries, 8.6 in one brand is not 8.6 in another. This is the main reason a prescription names a brand and why a seller cannot substitute an “equivalent”.
Power, and why it differs from your glasses
Power is measured in dioptres, in 0.25 steps. A −2.00 is mildly short-sighted; −8.00 is strongly so.
Here is the part that surprises people. From roughly ±4.00 dioptres upward, the contact lens power differs from the spectacle power, because the lens sits on the eye instead of 12 mm away. This is vertex distance compensation.
A −6.00 spectacle prescription typically becomes about −5.50 in a contact lens. Copying the number across leaves you genuinely over-corrected, with eye strain and headaches to match.
Which is why a glasses prescription cannot be used to order contacts — see the full explanation.
If you have astigmatism
CYL and AXIS appear. CYL is how much astigmatism, AXIS is which direction it runs.
The complication specific to contacts is that a toric lens must stay oriented on an eye that blinks. Designs use ballast or thin zones to keep the axis stable. If your vision goes in and out of focus through the day, that is a stabilisation problem — say it that way in the appointment rather than asking for a stronger prescription.
Note also that toric ranges are narrower than spherical ones, particularly in dailies. Your exact cylinder and axis may not exist in every product.
If you are over 40
An ADD power appears for near work, often written LOW, MED or HIGH rather than a figure, and it frequently differs between eyes. You may also see a dominance note — the fitter recording which eye leads for distance. See multifocal lenses.
The brand name is legally binding
This is not a manufacturer preference on the prescriber’s part. Under the FTC Contact Lens Rule a seller may not substitute a different brand or manufacturer than the one written.
If a retailer tells you they will ship a comparable lens instead, that is a violation, and the practical risk is real: the substitute may match on power while differing on base curve, diameter, material and oxygen transmission. You can report it.
The expiry date
One year is the federal minimum, not the universal rule — several states require two. The clock runs from issue, not from your first order. See expiration explained and the state-by-state breakdown.
Once it lapses, no legitimate seller can fill it — options in ordering with an expired prescription.
Three things worth doing with the document
- Photograph it the day you receive it. You are entitled to a copy automatically, without asking and without extra charge — see your rights.
- Check the issue date and set a reminder at ten months, not twelve.
- Keep it where you shop. Any legitimate retailer will verify it with your prescriber — see how verification works.
One last note: your contact lens prescription will not work for ordering glasses either. It has no pupillary distance and the powers are vertex-compensated. They are two separate documents with two separate expiry dates.
Sources
- FTC Contact Lens Rule, 16 CFR Part 315 — §315.2, §315.3, §315.5(f), §315.6(a)
- Clinical references on vertex distance compensation and base curve selection
General information. Your optometrist determines which lens suits your eye. Reviewed by Dr. Teresita Bollar, OD.
