Have you ever looked at your contact lens prescription — really looked at it? Beyond glancing at all those numbers and abbreviations to order contacts online, most of us hardly pay any attention to them.

Your contact lens prescription (Rx) is actually full of fascinating details about the unique composition of your eyes, and your vision!

Learning how to read your contacts prescription gives you insight into your eye health, and can even explain why some contacts you’ve tried may not have been comfortable. It can also help you out of a bind if you’re away from home and need an emergency pair of lenses (but we hope you always carry a spare pair of daily contacts!).

Let’s break out the key parts of your contact lens prescription, so you can get to know your eyes a little better.

Key abbreviations in your Rx

Contact lens Rx: right, left, power, and shape

The first part of your prescription identifies which eye is which, how much correction is needed to help you see clearly, and the physical measurements of your eyes.

OD and OS

OD is short for oculus dexter, which is Latin for ‘right eye.’ OS is the abbreviation for oculus sinister, which sounds like your eye’s evil twin — but means ‘left eye.’ OD always comes first on a contact lens prescription because that’s the eye your eye doctor examines first when facing you, looking left to right.

You might also see OU. Oculus uterque, or ‘both eyes,’ is used only if both of your eyes have the same prescription.

PWR or SPH

These abbreviations for ‘power’ or ‘sphere’ refer to your vision correction strength — how much correction each eye needs to achieve 20/20 vision.

Lens power is measured in diopters (D), a unit that specifically describes the optical power of a lens to bring objects into focus. If you see a plus sign (+) before the strength, it means that you have hyperopia, or farsightedness, in that eye. A minus sign (-) indicates myopia, or nearsightedness.

Most contacts are manufactured in 0.25D increments; for example, -1.25, -1.50, -1.75, and so on. If you have a high degree of nearsightedness (-6.00 and above), you can get contacts in 0.50D increments. The same is true for farsightedness, but the increments switch from 0.25D to 0.50D at +4.00.

In short: the higher the number on the (+) or (-) side, the more corrective power your eyes need.

If you have one eye that doesn’t need vision correction, you might see PL (plano) on your prescription.

BC stands for ‘base curve,’ or the curve of the lens. Base curve is measured in 0.1mm increments and is usually between 8 and 9, with higher numbers indicating a flatter lens.

If the base curve is off, your lenses won’t fit properly, and they might slip — or even (yikes!) pop out of your eye. That’s why you need your prescription to order contacts online!

DIA

This number is the diameter or width of the lens, also measured in millimeters. It’s important that your lenses have a diameter large enough to properly cover your cornea, but not too large for a comfortable fit. The most common contact lens diameters range from 13.6mm to 14.5mm.

Contact lens Rx: astigmatism

If you have astigmatism — and yes, you can wear contacts with astigmatism! — your contact lens prescription will have a few additional sections that explain the strength of your astigmatism, and where it’s located in your eye.

CYL

This is short for ‘cylinder,’ a negative value measured in increments of 0.50D that refers to the extra correction needed for astigmatism (distortion and blurring caused by an irregularly shaped cornea).

AXIS

‘Axis’ is an angle between 0 and 180 degrees, and indicates where exactly on your cornea the irregularity is located. On a toric lens — contacts designed for astigmatism — the number pinpoints where the astigmatism correction needs to sit to prevent blurred vision.

Aveo is proud to be the first and only subscription contacts brand offering daily toric lenses! Our Joy contacts for astigmatism feature exclusive SteadyView toric design for extra comfort and stability. Like all Aveo contacts, Aveo Joy lenses feature all-day hydration, best-in-class breathability, and built-in UV block — at a much more affordable price than other comparable premium toric lenses.

Monovision and multifocal contacts prescriptions

If you need both nearsighted and farsighted vision correction, you’ll have a prescription for multifocal lenses. Multifocal contacts have different prescriptions in different areas of the lens for near, far, and middle distance. Because the prescription varies throughout the lens, multifocals can take some getting used to.

If you don’t want to wear multifocals, you can choose monovision — wearing a lens for nearsightedness in one eye, and for farsightedness in the other.

Many people who are candidates for multifocal lenses or monovision opt instead to wear contacts for nearsightedness, and reading glasses for up-close vision correction.

ADD

Presbyopia is a common age-related process where your eyes gradually lose the ability to focus on nearby objects. If you have presbyopia, you likely have a prescription for multifocal lenses. ADD stands for ‘add power’ — it’s the additional level of correction required to give you clear vision for reading up close. ADD is between +0.50D and +3.00D, although some prescriptions will simply show low, medium, or high for this measurement.

D & N

A ‘D’ indicates your dominant eye that the prescription is based on, and ’N’ refers to your non-dominant eye.

Are contacts and glasses prescriptions the same?

Prescriptions for contacts and glasses have some similarities, but they aren’t interchangeable. You can’t directly convert your glasses prescription to contacts. Here’s what you need to know:

Both contacts and glasses prescriptions have OD, OS, and PWR/SPH
If you have astigmatism, both contacts and glasses prescriptions will have CYL and AXIS
Contacts sit on your eyes, so the prescription will include BC and DIA
Glasses sit a small distance from your eyes, so the prescription has to compensate for that distance — it is typically slightly stronger
Because of this difference in strength, mild astigmatism or presbyopia may be noted in your eyeglasses prescription, but not in your contact lens prescription (people with very mild astigmatism can often wear non-toric lenses)
Glasses prescriptions include PD or ‘pupillary distance’ — the distance between your two pupils, measured in millimeters
Glasses prescriptions can include ‘prism,’ which specifies measurements in diopters to compensate for eyes that aren’t aligned
Glasses prescriptions often have recommendations for lens coatings (i.e., polarized, transition, or blue light–blocking) and designs

Now that you know how to read your contact lens prescription, you can understand why it’s so important to have a current prescription — and why brands are required to ask for your prescription details when you purchase contacts online.