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Back Vertex Power Calculator

This calculator is designed to convert eyeglass prescriptions to contact lens prescriptions using the back vertex power equation. It allows inputs in both minus and plus cylinder formats, automatically converting plus cylinder prescriptions to minus cylinder before calculation. The tool defaults to a 12mm vertex distance but allows users to adjust this value to suit their purposes. The output is a vertexed prescription rounded to the nearest 0.25 diopter.

Back Vertex Power Equation:

Back Vertex Power Equation

BVPcontacts​ = BVPspectacles​​/ (1−(vertex distance in meters × BVPspectacles​))

Related Links:

  1. Printable Vertex Table

Diopter-Radius Converter

Result:

To convert between radius of curvature in millimeters (mm) and diopters (D), use the following formula:

D = (n − 1) × (1000 / R)

Where D is the dioptric power, R is the radius of curvature in millimeters, and n is the refractive index of the surface.

You can also print our Diopters to Radius Conversion Chart (PDF) for later reference.

Safe Daily Dose Calculator

The safe daily dosage of Plaquenil (hydroxychloroquine) is calculated based on 5.0 mg per kilogram of actual body weight, as recommended by the 2016 American Academy of Ophthalmology guidelines.

Cumulative Dose & Toxicity Calculator

🧠 Why the 1000 Gram Limit?
The American Academy of Ophthalmology guidelines recommend increased screening vigilance once a patient has exceeded a cumulative dose of 1000 grams of hydroxychloroquine. This threshold marks a significant increase in the risk of permanent retinal toxicity, even if the drug is later discontinued. Other major risk factors that warrant closer monitoring include a high daily dosage (>5.0 mg/kg), use for over 5 years, the presence of kidney or macular disease, and concurrent tamoxifen use.

Plus/Minus Cylinder Converter