About
VertexLensCalc is an independently operated educational website with tools for understanding prescription power and contact lens costs. Content is published under the VertexLensCalc name. We are not an eye care practice, and our content and tools have not been clinically reviewed by an eye care professional.
What the Vertex Tools Calculate
The vertex calculators estimate how optical power changes between the spectacle lens plane and the contact lens plane. The main calculator works in two directions: glasses to theoretical contact power using Fc = Fs / (1 − d × Fs), and contact power to a theoretical glasses equivalent using Fs = Fc / (1 + d × Fc). Distance d is in meters. The toric calculator focuses on glasses-to-contacts conversion. Neither direction reconstructs a complete prescription.
For astigmatism, the two principal meridians are S and S + C. We convert both and reconstruct cylinder from the difference between their raw results. Plus-cylinder inputs are transposed to minus-cylinder notation. Axis is required when CYL is not zero; the tools do not invent a missing axis.
The assumed vertex distance is 12 mm, with inputs from 10 to 14 mm. Results show raw values and a 0.25 D display increment. This increment is our display convention; it is not an ISO or ANSI certification and does not represent every product’s available powers.
Spherical Equivalent Method
The spherical equivalent calculator uses SE = SPH + CYL/2 to average the two principal meridian powers at the same lens position. Axis and vertex distance are not used. The exact result is shown separately from the nearest 0.25 D display; halfway values round away from zero. An average power does not preserve the full astigmatic correction.
Cylinder Transposition Method
The cylinder transposition calculator rewrites the same optical correction in plus- or minus-cylinder notation: new SPH = SPH + CYL, reverse the CYL sign, and shift Axis by 90° within 1–180°. It does not apply vertex conversion or round the powers. With zero CYL, SPH stays unchanged and Axis is not applicable. Worked examples and method references appear on each tool page.
What the Tools Cannot Determine
A mathematical power conversion does not issue a prescription or determine lens fit. Base curve, diameter, material, product availability, lens rotation and current eye health require separate consideration by an eye care professional.
The vertex tools do not model front/back surface power, frame tilt, face form, over-refraction or prism. The cost tools provide illustrative budget estimates; actual prices, service packages and insurance benefits depend on the provider, product and plan.
How Content Is Prepared
AI tools assist with drafting and editing. VertexLensCalc is responsible for publishing the content. Source links and calculation assumptions are provided to make explanations easier to check. Source attribution and automated drafting do not establish medical expertise or clinical review.
When content is changed, the update date should reflect that change. A page update does not mean that a clinician reviewed it or that every quoted price is a current provider offer. Pricing examples need their own date and scope.
Calculation References
Manufacturer tools illustrate the distinction between theoretical vertex conversion and product-specific fitting:
- ACUVUE Simplifit describes vertex calculation across the power meridians.
- CooperVision Toric Calculator separates theoretical powers from available lens parameters and professional fitting.
These links are method references. VertexLensCalc is independent of these manufacturers and does not claim their endorsement.
Contact and Corrections
If you find a calculation, source or content error, email contact@vertexlenscalc.com. Include the page and the issue you noticed; avoid sending personal medical records. Our disclaimer explains the educational scope of the site.