Glasses → Contacts
Spherical Equivalent and Vertex Distance Calculator
Two numbers stand between a glasses prescription and a contact lens power. The spherical equivalent collapses sphere and cylinder into one spherical value: sphere plus half the cylinder, rounded to 0.25 D. The vertex distance conversion then corrects that power for the 12 mm a lens moves when it goes from the frame to the eye, which matters from about 4.00 D upward. Both calculators are below, with the formulas shown, so you can check the result with your optometrist before ordering.
Page reviewed 2026-09-15. Formulas unchanged since the page was built.
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.
Always verify converted values with a licensed optometrist or ophthalmologist before ordering contact lenses. Professional fitting confirms base curve, diameter, and corneal health.2
Spherical Equivalent
Spherical Equivalent Calculator
The spherical equivalent is sphere + (cylinder / 2), rounded to the nearest 0.25 D. For -3.00 -1.00 x 180 it is -3.50 D. It is the one spherical power that best stands in for a prescription with astigmatism, and the power a spherical soft lens can correct when the cylinder is small. Enter the values exactly as written, signs included.
Spherical equivalent
-3.50 D
Sphere + (cylinder / 2), rounded to the nearest 0.25 D. Enter the values exactly as written on the glasses prescription, including the sign. With a cylinder of 1.00 D or more, a spherical equivalent lens usually leaves noticeable blur; that is the range where a toric lens is the normal fit. See the toric lens calculator.
A spherical equivalent lens leaves the astigmatism uncorrected. From about 1.00 D of cylinder the blur is usually noticeable and a toric lens is the normal fit; the toric lens calculator covers that case.
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
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.
- 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.
- Enter each value in the corresponding converter field. Adjust vertex distance only if your eye care provider documented a custom measurement.
- Review the calculated contact lens powers, confidence guidance, and toric recommendations provided below the results panel.
- Schedule or confirm a contact lens fitting appointment to verify base curve (BC), diameter (DIA), and lens material suitability.7
- 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
- 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
- Centers for Disease Control and Prevention. Contact Lens Fast Facts. Accessed October 5, 2025. Available at: https://www.cdc.gov/contactlenses/fast-facts.html
- Review of Optometry. How to Calculate Vertex Distance. 2024. Available at: https://www.reviewofoptometry.com/article/how-to-calculate-vertex-distance
- Council on Optometric Practitioner Education. Vertex Conversion Tables for High Powers. COPE Course 81827-GO, 2023.
- Contact Lens Spectrum. Managing Astigmatism with Toric Soft Lenses. February 2024.
- Gunderson G, Patel S. Clinical thresholds for toric contact lens prescribing. J Cataract Refract Surg. 2022;48(7):845-852.
- American Optometric Association. Guide to Contact Lens Fitting Appointments. 2023.