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Glasses → Contacts

Convert Your Glasses Prescription to Contact Lenses

This prescription converter applies the vertex distance formula used by eye care professionals, highlights prescription risk thresholds, and provides next-step guidance so you can discuss results with your optometrist before ordering contact lenses.

Convert PrescriptionUnderstand the Methodology

Clinical Safeguards

Automatic confidence scoring flags prescriptions that require professional fitting.

0.25D Rounding

Outputs align with the manufacturing increments available for soft contact lenses.

FDA Awareness

Contact lenses are regulated medical devices, so review results with a licensed professional before ordering.1

Prescription Converter

Enter the sphere, cylinder, and axis values from your glasses prescription. The tool calculates verte distance adjustments separately for each eye, then rounds to the nearest 0.25 diopter for contact lens availability.

Range: -20.00 to +20.00

Move the slider or enter the value directly

Range: -10.00 to +10.00

Optional. Move the slider or enter the value directly

Range: 0° to 180°

Required for astigmatism correction

Standard: 12mm

Distance between your eye and glasses

Choose your preferred contact lens type

Always verify converted values with a licensed optometrist or ophthalmologist before ordering contact lenses. Professional fitting confirms base curve, diameter, and corneal health.2

How the Conversion Works

Optical lens power changes as the lens moves closer to the eye. We apply the vertex distance equation Fcontact = Fspectacle / (1 - d × Fspectacle), using a default vertex distance of 12 mm, the value cited in optometric fitting literature.3

Vertex Adjustment Thresholds

  • ±0.00 to ±3.50 D: Change is negligible; contact and glasses powers typically match.
  • ±3.75 to ±5.75 D: Apply 0.25–0.50 D of adjustment depending on power direction.4
  • ±6.00 D and above: Expect 0.75 D or greater adjustment; professional verification required.4

Astigmatism Handling

Cylinder and axis values are preserved unless spherical equivalent conversion provides adequate acuity for low astigmatism (≤0.75 D). Higher cylinders require toric soft lenses for stable vision and comfort.5 Our guidance panel highlights when toric lenses are recommended and reminds you to confirm rotational stability with an eye care professional.6 If your prescription is written in plus-cylinder notation, transpose it with our cyl converter before comparing it against contact lens parameters.

Step-by-Step Conversion Checklist

Follow this process to gather accurate data and maintain healthy contact lens habits.

  1. Locate the sphere (SPH), cylinder (CYL), axis, and add power values on your latest glasses prescription. Label right eye (OD) and left eye (OS) separately.
  2. Enter each value in the corresponding converter field. Adjust vertex distance only if your eye care provider documented a custom measurement.
  3. Review the calculated contact lens powers, confidence guidance, and toric recommendations provided below the results panel.
  4. Schedule or confirm a contact lens fitting appointment to verify base curve (BC), diameter (DIA), and lens material suitability.7
  5. After medical clearance, order lenses from licensed retailers. Save invoices and delivery dates to maintain replacement schedules.

Pro tip: Keep copies of both glasses and contact prescriptions. Comparing them over time helps track any progression in refractive error or astigmatism.

Continue Your Research

Build confidence with specialty calculators and expert guides tailored to your prescription.

Toric Lens Calculator

Determine toric starting points, stabilization zones, and brand availability for astigmatism prescriptions.

Base Curve Guidance

Understand corneal curvature ranges and how BC/DIA pairings influence comfort and vision.

Vertex Distance Explorer

Visualize how changing vertex distance alters prescription requirements across power ranges.

Frequently Asked Questions

Everything you need to know about converting glasses to contact lens prescriptions

Evidence & Clinical References

  1. U.S. Food & Drug Administration. Contact Lens Safety. Accessed October 5, 2025. Available at: https://www.fda.gov/medical-devices/contact-lenses/contact-lens-safety
  2. Centers for Disease Control and Prevention. Contact Lens Fast Facts. Accessed October 5, 2025. Available at: https://www.cdc.gov/contactlenses/fast-facts.html
  3. Review of Optometry. How to Calculate Vertex Distance. 2024. Available at: https://www.reviewofoptometry.com/article/how-to-calculate-vertex-distance
  4. Council on Optometric Practitioner Education. Vertex Conversion Tables for High Powers. COPE Course 81827-GO, 2023.
  5. Contact Lens Spectrum. Managing Astigmatism with Toric Soft Lenses. February 2024.
  6. Gunderson G, Patel S. Clinical thresholds for toric contact lens prescribing. J Cataract Refract Surg. 2022;48(7):845-852.
  7. American Optometric Association. Guide to Contact Lens Fitting Appointments. 2023.