How do you read a contact lens prescription?
A contact prescription is a short list of parameters, not a mystery. Read the eye labels, then the five numbers that show up most often. You can decode a typical card in a couple of minutes.
The eyes
- OD means right eye (oculus dexter).
- OS means left eye (oculus sinister).
- Some cards skip Latin and just say R and L.
Each eye can have different numbers. That is normal.
The numbers that usually appear
PWR or SPH (power / sphere). The strength. Negative means nearsighted (most wearers). Positive means farsighted. Example: -3.25 is a common nearsighted power. Each eye often differs.
BC (base curve). How curved the lens is, in millimeters, often around 8.4 to 8.7. It has to match how the lens was fitted on your eye. Wrong BC means a lens that moves or sits poorly when you blink.
DIA (diameter). Width of the lens in millimeters, often 14.0 to 14.5. Also set by the fitting, not by preference.
CYL and AXIS (only with astigmatism). CYL is the extra cylinder correction. AXIS (0 to 180) is the angle. If these are on your card, you need toric lenses. Regular spherical lenses will not fully clear that blur.
ADD (only for multifocals). Extra near power, often written LOW, MED, or HIGH. Most people under 40 do not have this yet.
Brand or lens name on the card matters too. Contact prescriptions name a specific product because power, base curve, diameter, and material were fitted together.
Two gotchas
- A glasses prescription is not a contact prescription. Glasses sit about 12mm from your eye. Contacts sit on it. Powers can differ, and glasses cards have no BC or DIA. You cannot convert one into the other yourself. That is the fitting's job. More on expired cards in what happens if your contact prescription is expired.
- You cannot freely swap brands. Changing brand or material legally needs your prescriber's OK, even if the power looks similar.
Expiration
US contact prescriptions typically last one to two years, depending on your state. After that you need a new contact lens fitting. Annoying, and also the checkup a lot of contact wearers skip until something feels wrong.
If you can find your PWR and tell OD from OS, you have the literacy buying contacts requires. Sellers still verify the full parameter set before they ship.
FAQ
Can you use a glasses Rx to buy contacts?
No. You need a contact lens fitting and a contact prescription that lists brand, power, base curve, diameter, and expiration.
What if OD and OS are different?
That is common. Order the parameters written for each eye. Do not average them.
What do BC and DIA mean in practice?
Fit numbers. They describe the shape and size of the lens your doctor fitted, not optional upgrades.
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