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More than 45 million Americans and 3.5 million Canadians wear contact lenses to correct their vision. 

While roughly 1 in 10 of us needs contacts to help us see clearly, not everyone knows they need to have a prescription for their lenses containing information specific to their eyes. 

But a contact lens prescription isn’t just a nice-to-have - It’s critical for your eye health and there are risks associated with not having the right prescription for your lenses. 

Blurred vision 

Contact lenses are meant to improve your vision. Wearing them with the wrong prescription will cause vision impairment. Wearers may also experience eye strain, headache, and dizziness when using the wrong prescription. It’s also worth noting that experiencing blurred vision while wearing your contact lenses is one sign of an expired prescription.


Without the right prescription, it’s difficult to get a perfect fit for your contact lenses. Everyone’s cornea (the front part of the eye) is shaped differently. It requires a number of different measurements to ensure your lenses are fitted properly. If your contacts are too tight, it can result in hypoxia means your cornea is not getting enough oxygen. If they’re too loose, your contact lenses may move around and cause corneal scratching, which can lead to severe eye damage. 

Corneal ulcers 

Wearing the wrong prescription with your contacts can also lead to corneal ulcers. Eye ulcers cause scarring on the eye’s surface, resulting in light sensitivity and severe tearing. Neglecting corneal ulcers can lead to permanent vision loss. 

What is a contact lens prescription? 

glasses and contacts

Your contact lens prescription contains all of your vision correction specifications and tells you which type of contact lenses you need. This includes information about both the power of the lens and the size of the lens. Each of your eyes may have different vision problems that need correcting, so you may have different prescriptions for each eye.  

These prescriptions are given out by an optometrist after an eye exam and are valid usually for one year.

Can I use my glasses prescription for my contact lenses?

Unfortunately, no. These two prescriptions are not the same. A contact lens prescription will provide additional information about the size of the lens. What’s more, the power will also be different between glasses and contacts, because while a contact lens sits directly on the surface of your eye, glasses sit about 12mm in front of it. So a contact lens prescription will account for that difference. 

What do the letters and numbers on a contact lens prescription mean? 

Let’s break down what all those coded numbers and letters on your prescription really mean: 

OD and OS

OD and OS simply mean left eye and right eye. They come from the Latin terms oculus dexter and oculus sinister, respectively. Another letter combo you may see is OU which stands for oculus uterque, or both eyes, which indicates the measurement applies to both eyes.


PWR refers to refractive power and may also be referred to as SPH for sphere. This number, measured in diopters, reflects the correction needed to achieve 20/20 vision (or as close to 20/20 vision as possible). 

If this is a negative number, it means you are nearsighted (also called myopia). If it’s a positive number, you are farsighted (also called hyperopia). The further the number is from zero in either direction, the stronger your prescription. Many people have different prescriptions in each of their eyes. 


BC stands for base curve and indicates the curvature of the lens. It’s measured in millimeters and usually falls between 8 and 10. This measurement ensures a snug fit for your lens as it matches the shape of your cornea. Some lenses won’t have this measurement as they only come in one base curve size. 


This measurement stands for diameter and covers the distance across the surface of your lens. Also measured in millimeters, it usually sits between 13 and 15 and determines where the contact lens will set on your eye. This measurement is especially important because your lens may be uncomfortable and scratch your eye if it's not right. 

Toric contact lenses

Toric contact lenses are designed to correct astigmatism issues that cause a different curvature of the cornea or lens in your eye. You’ll need specific measurements for these, which include: 


This is the cylinder value and is the amount of power needed to correct your astigmatism. This number usually sits between -4 and +4. 


This measures the orientation of the cylinder value on the lens and is calculated in degrees, between 0° and 180°.

Bi-focal or multi-focal lenses 

Those who are both near and farsighted will require bi-focal or multi-focal lenses. These are often required for those who suffer from presbyopia, the gradual loss of your eyes' ability to focus on nearby objects, which tends to happen starting in middle age. 

Bi-focal and multi-focal lenses can help with this condition by providing vision correction for both distance and near vision. 

You’ll need a specific measurement for these lenses called ADD. ADD, which stands for added power, is measured in diopters and indicates the added magnifying power needed in parts of the lens to help with seeing up close. 

It’s easy to see just how many specific measurements are required in a contact lens prescription to ensure they’re the right fit for your eyes.

Getting help with your contact lens prescription

At VisionPros, if you don’t have a copy of your valid prescription, you can still place your order by providing your eye doctor's information. Once your prescription has been validated by your eye doctor, we’ll ship your order.

Unfortunately, we can’t ship orders with expired or invalid prescription information. If you don’t have a valid prescription, it’s important to see your local optometrist to have your yearly comprehensive eye exam to ensure you continue to see well and have great ocular health.

If you have any further questions, please contact our customer service team at 1-800-404-7317 or email info@visionpros.com.