How to Read a Contact Lens Prescription

Buy contacts online - learn how to read a contact lens prescription to order the right lenses.

Do you know what your contact lens prescription is?

I'm always amazed when I ask someone what power their lenses are or what base curve and diameter lenses they wear? And they haven't a clue what I mean. Most of my friends don't even know which brand of contact lens they wear!

If that sounds like double dutch to you too, then you need to learn how to read a contact lens prescription if you want to order contacts online. And that means understanding what the various bits of your prescription stand for.

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I'll start with a simple contact lens prescription, common to people who do NOT have astigmatism, which complicates the prescription.

read contact lens prescription Typical Prescription for Contact Lenses


  • Here's what the various numbers mean:

  • OD = Right Eye.

  • OS = Left Eye (if both eyes are the same power you may see OU instead).

  • Power = the precription "power" of your lens, either a plus or a minus number depending on whether you are short (minus) or long sighted (plus).

  • BC = Base curve, the curved shape of lens, usually a number between 8.0 and 9.5.

  • Dia = Diameter, the width of the lens, usually between 13.5 and 14.8; if wider, the lens will cover a larger portion of the eye, for example theatrical contact lenses, which have a diameter of 18-22.0.

contact lens prescription Contact Lens Prescription

If you want to buy contacts online and you can't find a copy of your precription, don't panic as the information can be found on the box containing your contact lenses.

In this picture, for example, there are two different types of contact lenses, one a daily disposable lens called Acuvue Moist and the other a 2 weekly disposable lens, Acuvue Advance.

The BC and Dia are clearly marked and the Power is denoted by the letter D, which stands for dioptric power. You will also see a use by date on the box.

If you have astigmatism, then your prescription will contain two additional fields, CYL for Cylinder and Axis. This means that your lenses effectively have two different powers, one to correct your short or long-sight and the other the irregular curve of your eye (the astigmatism).

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If you DON"T need contact lenses for correcting your vision, you still need to know how to read a contact lens prescription, if for instance you want to order discount colored contact lenses online for fashion. Why? Because without one, you won't know what size of lens to order.

Different brands come in varying sizes of Base Curve and Diameter, plus the same BC in one brand may not be exactly the same in another brand, so it's not a one-size-fits-all situation.

Basically your optician will FIT a lens that is most appropriate to the curvature of your eye and choose the best brand accordingly.

Tip: Once your lenses have been fitted by an optician, learn how to remember your prescription. An easy way to do this is to keep the tab from the contact lens box in your purse so you never forget!

In the picture above, you'll see that the base curve and diameters are slightly different between the two lenses. I can wear both sizes as the lenses are made by the same brand and they are only marginally different in size. I can't however wear colored lenses from a different brand called Freshlook, as they have a Diameter of 14.6 - they're simply too big for me.

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