Contact Lens to Glasses Prescription Converter
Use this calculator to estimate your glasses prescription based on your current contact lens prescription. This conversion is crucial due to the difference in vertex distance between contact lenses (which sit directly on the eye) and glasses (which sit a short distance away from the eye).
Your Contact Lens Prescription
Typical glasses vertex distance is 10-14mm. Consult your optician if unsure.
Right Eye (OD)
Left Eye (OS)
Estimated Glasses Prescription
Right Eye (OD)
Sphere: — D
Cylinder: — D
Axis: — °
Left Eye (OS)
Sphere: — D
Cylinder: — D
Axis: — °
Understanding Your Contact Lens to Glasses Prescription Conversion
When you receive an eye prescription, it's typically for glasses. However, if you wear contact lenses, your contact lens prescription will often differ from your glasses prescription. This isn't because your vision has changed, but due to a critical optical principle known as "vertex distance."
What is Vertex Distance?
Vertex distance is the space between the back surface of a corrective lens (either glasses or contacts) and the front surface of your eye (cornea). Contact lenses sit directly on the cornea, meaning their vertex distance is essentially zero. Glasses, on the other hand, sit a small distance away from your eye, typically between 10 to 14 millimeters (mm).
This seemingly small difference becomes optically significant, especially for higher prescriptions (generally above +/- 4.00 diopters). The effective power of a lens changes with its distance from the eye. A lens that corrects your vision perfectly when sitting on your eye will have a different effective power when moved a few millimeters away.
Why the Conversion is Necessary
- Myopia (Nearsightedness – Minus Lenses): For myopic individuals, contact lenses are stronger (more minus) than the equivalent glasses prescription. When a minus lens is moved further from the eye (like glasses), its effective power becomes weaker. Therefore, your glasses prescription will typically be less minus than your contact lens prescription.
- Hyperopia (Farsightedness – Plus Lenses): For hyperopic individuals, contact lenses are weaker (less plus) than the equivalent glasses prescription. When a plus lens is moved further from the eye, its effective power becomes stronger. Thus, your glasses prescription will typically be more plus than your contact lens prescription.
The calculator above uses a standard formula to account for this vertex distance, providing an estimated glasses prescription.
Components of Your Prescription
Both contact lens and glasses prescriptions typically include:
- Sphere (SPH): This indicates the main power of the lens, correcting for nearsightedness (-) or farsightedness (+). It's measured in diopters (D).
- Cylinder (CYL): This corrects for astigmatism, an imperfection in the curvature of your eye. It's also measured in diopters.
- Axis: This number (between 1 and 180 degrees) indicates the orientation of the cylinder power.
- OD (Oculus Dexter): Refers to your right eye.
- OS (Oculus Sinister): Refers to your left eye.
While the Sphere power is significantly affected by vertex distance, the Cylinder and Axis values usually remain the same or require only minor adjustments by an eye care professional, especially for very high astigmatism.
How to Use the Calculator
- Enter Vertex Distance: The default is 12mm, which is a common average. If you know your specific glasses' vertex distance, enter it.
- Input Contact Lens Prescription: Carefully enter the Sphere, Cylinder, and Axis values for both your Right Eye (OD) and Left Eye (OS) exactly as they appear on your contact lens prescription.
- Click Calculate: The calculator will then display the estimated equivalent glasses prescription.
Important Considerations
This calculator provides an estimation and should not replace a professional eye examination. Several factors can influence the final glasses prescription, including:
- Individual Eye Anatomy: Each person's eyes are unique.
- Lens Design: Different lens materials and designs can subtly affect power.
- Doctor's Discretion: Your optometrist or ophthalmologist will fine-tune your prescription based on your comfort, visual acuity, and specific needs.
- Prescriptions below +/- 4.00 D: For lower prescriptions, the vertex distance effect is often considered negligible, and the contact lens and glasses prescriptions may be identical or very close. However, this calculator applies the formula universally for accuracy.
Always consult with your eye care professional to obtain an accurate and personalized glasses prescription.
Example Conversion
Let's say your contact lens prescription for your Right Eye (OD) is:
- Sphere: -8.00 D
- Cylinder: -1.00 D
- Axis: 180°
Using a typical vertex distance of 12mm (0.012 meters):
The formula for sphere conversion is: P_glasses = P_contacts / (1 - d * P_contacts)
P_glasses = -8.00 / (1 - 0.012 * -8.00)
P_glasses = -8.00 / (1 + 0.096)
P_glasses = -8.00 / 1.096
P_glasses ≈ -7.30 D
Rounded to the nearest 0.25 D, this would be -7.25 D or -7.50 D depending on the exact rounding. The calculator will handle this precision.
The Cylinder and Axis would typically remain -1.00 D and 180° respectively.
So, your estimated glasses prescription for OD would be approximately: Sphere -7.25 D, Cylinder -1.00 D, Axis 180°.