How to Convert Glasses Prescription to Contacts?

How to Convert Glasses Prescription to Contacts?

While both glasses and contact lenses are designed to correct refractive errors such as astigmatism, nearsightedness, and farsightedness, the prescriptions for the two are not interchangeable. The difference lies in the fact that glasses sit approximately 12mm from your eyes, whereas contact lenses are placed directly on the surface of your eyes. This discrepancy in distance changes the way light enters your eyes, thus necessitating a different prescription for glasses compared to contacts.

Aside from the basic correction parameters, a contact lens prescription incorporates further specifications. These include the base curve and diameter, which are crucial for achieving a precise fit on the eye's cornea. With that in mind, it's essential to understand how to convert your glasses prescription to contacts accurately.

Understanding Your Glasses Prescription

Before converting your glasses prescription to contacts, you must understand how to read a glasses prescription. We won't be delving into the details of understanding your glasses prescription in this article, as we have thoroughly covered it in a previous post. Nevertheless, it's crucial to have a clear understanding of your glasses prescription before you proceed. This knowledge will empower you to make informed decisions about your eye care.

Now, let's move forward and discover how to convert your glasses prescription to contacts.

Glasses and Contact Lens Differences

If you compare an eyeglass prescription and a contact lens prescription, you'll notice a clear difference. The glasses prescription will have a cylinder and axis value, which you won't usually find on a contact lens prescription unless you're getting a toric or multifocal lens.

Contact lenses require specific measurements that are tailored to fit your eyes. These measurements, including the base curve and diameter of the lens, ensure a customized fit that accounts for the shape and size of your eyes. Unlike glasses prescriptions, which don't account for these details, contact lenses need to be precisely matched to your eyes to provide optimal vision correction.

Vertex Conversion Chart for Contact Lenses

Opticians utilize a vertex conversion chart to simplify eye tests and calculations, making the process easier for everyone. This chart contains various figures for different lens powers. If you happen to be nearsighted, your prescription will include a negative or minus (-) sign. Conversely, if you're farsighted, it will have a positive or plus (+) sign. Rest assured, opticians have you covered with this helpful tool!

How Vertex Conversion Charts Work?

Vertex conversion charts are easy to use. All you need to do is find the relevant columns and understand their meanings. The column labeled 'glasses lens power' corresponds to your current glasses prescription. If the power is negative, you can find the conversion on the left side of this column. On the other hand, if the power is positive, your converted figure will be on the right.

It is worth noting that vertex conversion charts typically start at a specific range of eyeglass prescriptions. This is because they usually exclude lower lens powers where the converted power is not significantly different from the original prescription. If the difference obtained from the vertex distance conversion formula is less than 0.25, it will not have a noticeable impact on your visual acuity. In simpler terms, if your prescription falls within this range, you can use the same power as your glasses prescription unless you have astigmatism. In such cases, additional measurements are required to account for the variations in the shape of your cornea.


In conclusion, understanding the process of converting glasses prescription to contact lenses is crucial to ensure accurate vision correction. Vertex conversion charts are a valuable tool in achieving this, as they provide a simple and reliable way to calculate the proper contact lens power. By following the instructions and using the chart correctly, you can confidently make informed decisions about your eye care needs. Always consult with your optician if you have any doubts or concerns about your prescription or the conversion process.

Frequently Asked Questions - FAQs

Is There a Difference Between Glasses and Contact Lens Prescriptions?

Although eyeglass and contact lens prescriptions may appear similar to each other, there are significant differences between the two that can result in variations in visual acuity based on your refractive error. It's important to remember that eyeglasses are positioned approximately 1 cm away from your eyes, while contacts are placed directly on your eyes. This difference is crucial because the perceived power of your lens will be determined by how close or far it is from your eye.

What Happens If I Use the Same Prescription?

It is crucial to ensure that contact lens prescriptions are properly adjusted to avoid any discomfort, blurry vision, or eye strain. Improperly adjusted prescriptions can lead to issues such as overly strong contacts for nearsighted individuals or insufficiently strong lenses for farsighted individuals. Therefore, it is vital to be cautious and ensure that contact lens prescriptions are adjusted accurately to prevent adverse effects.

During an eye test for glasses, lenses are placed about 12-13 mm away from your eyes to simulate the space between your glasses and eyes. If the distance between your eyes and lenses increases, the lenses gain positive power, while reducing the distance results in more negative power. It's important to note that contact lenses sit directly on the eyes, eliminating the space between the eyes and lenses, which means they have more negative power than glasses.

Back to blog