Contact Lens Prescription Conversion Calculator
Contact Lens Vertex Calculator
Want to quickly convert your glasses prescription to a contact lens prescription? This easy-to-use calculator provides an instant estimate for colored and circle lenses.
Right Eye (OD)
Required field (in diopters)
Leave blank if you don't have astigmatism
Only required if you have cylinder value for astigmatism
Contact Lens Prescription
Left Eye (OS)
Required field (in diopters)
Leave blank if you don't have astigmatism
Only required if you have cylinder value for astigmatism
Contact Lens Prescription
Important Notes:
- This calculator provides an estimate using the vertex distance formula. Always consult with your eye care professional for accurate prescriptions.
- Contact lens prescriptions require additional parameters like base curve and diameter that are not calculated here.
- Results are rounded to the nearest 0.25 diopter for practical use.
- For prescriptions with high cylinder values (>2.00D), professional fitting is essential.
Note: This calculator is an estimate (For information only, not a prescription) and does not replace a professional eye exam.
What your results mean: Sphere, cylinder & axis
Sphere (SPH) shows the amount of nearsightedness or farsightedness being corrected. A minus value means you’re nearsighted, while a plus value means you’re farsighted.
Cylinder (CYL) indicates the level of astigmatism correction needed. Higher cylinder values require more precise fitting and may need toric lenses.
Axis represents the orientation of your astigmatism. Even if your cylinder value is small, the axis helps determine where the correction must be placed on the lens.
These three values guide your lens power, but they don’t determine lens fit—only the optical correction.
Additional measurements contact lenses require (BC, DIA, Brand)
Unlike glasses prescriptions, contact lenses also require Base Curve (BC) and Diameter (DIA) to ensure the lens fits the shape of your eye.
- BC determines how the lens curves and affects overall comfort and stability.
- DIA affects coverage and how the lens sits on your iris.
In addition, contact lens prescriptions are brand-specific because every manufacturer uses slightly different materials, curvatures, and designs. Even with the same numbers, not all lenses will fit the same way. A proper fitting ensures safety, clarity, and long-term comfort.
Common mistakes to avoid when converting your prescription
A common mistake is assuming your glasses prescription directly matches your contact lens power. Contact lenses sit on the eye, so values for higher prescriptions must be converted using vertex distance.
Another error is ignoring cylinder or axis values when astigmatism is present—standard spherical lenses won’t provide clear vision in these cases.
Users also sometimes overlook BC and DIA, which can result in an uncomfortable or unstable fit.
Lastly, relying solely on a calculator without a professional fitting can lead to incorrect lens selection. The calculator provides an estimate, but an eye exam ensures accuracy and long-term eye safety.
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Frequently Asked Questions
It’s a tool that transforms your eyeglasses prescription (sphere, cylinder, and axis) into an estimated contact lens prescription using the vertex distance formula.
Contact lenses sit directly on your eye (vertex distance zero), whereas glasses sit about 12 mm away, this distance affects effective lens power. The calculator corrects for that.
Yes, it factors in cylinder and axis values. However, if your cylinder is over 2.00 D, a professional fitting is strongly recommended.
No, power requirements stay the same across brands. But coverage differences like base curve and diameter may vary, so always choose the correct fit.
No, this is only an estimation tool. A proper eye exam and lens fitting are essential for safety, comfort, and accurate prescription.
Conversion becomes most important when your sphere power is -4.00 or higher (or +4.00 and above) because the difference between glasses power and contact lens power increases with stronger prescriptions. For astigmatism, cylinder values of –0.75 or higher benefit from accurate conversion.
No. Multifocal and progressive contact lenses require additional fitting parameters that cannot be calculated with simple sphere conversion. A professional contact lens exam is required to determine the correct add power and lens design
Vertex distance is the space between your glasses lenses and your eyes—usually around 12mm. Contact lenses sit directly on the eye, so this distance is removed. Higher prescriptions need power adjustments because the effective strength changes when the lens moves closer to the eye.
No. BC and DIA cannot be calculated from a glasses prescription. These measurements must be determined during a contact lens fitting because they depend on your eye shape, corneal curvature, and how the lens material fits your eye.
You can use it for a general estimate, but high cylinder values typically require toric contact lenses, which involve additional fitting steps. For strong astigmatism, a professional evaluation is the safest way to ensure clear vision and proper stability.
The optical conversion stays the same, but the fit can vary by brand because each manufacturer uses different materials, curves, and designs. Even with correct power, only certain brands may fit your eyes comfortably.
It’s recommended to update your contact lens prescription every 1–2 years, or sooner if you notice vision changes or discomfort. Even if the conversion doesn’t change, lens fit and eye health should be reassessed regularly.
Start by selecting lenses with a BC and DIA close to those recommended at your fitting exam. From there, choose a reputable, approved brand known for comfort and consistent quality. If you're new to wearing contacts, begin with a standard BC (around 8.6) and consult customer service if you're unsure.