Contact Lens Vertex Conversion Calculator

Work out contact lens vertex conversion instantly with clear inputs, formula shown and shareable results.

About Contact Lens Vertex Conversion Calculator

Applies the vertex distance formula to convert a spectacle prescription to the corneal plane, which matters above about 4 dioptres.

Formula

Vertex conversion

Fc = Fs / (1 - d x Fs), where d is the vertex distance in metres and Fs the spectacle power in dioptres

Worked example

With the default values below, Contact Lens Vertex Conversion Calculator returns Contact lens plane power of -5.597. Change any input above and every figure here is recalculated in your browser.

Inputs used

Spectacle plane poweraccepts -30–30 D: -6 D
Vertex distanceaccepts 5–20 mm: 12 mm

Results

Contact lens plane power: -5.597 D
Rounded to the nearest 0.25 D: -5.50 D
Change from the spectacle power: 0.403 D

Step by step

1Values used
2Vertex conversion
3Contact lens plane power= -5.597 D
4Rounded to the nearest 0.25 D= -5.50 D
5Change from the spectacle power= 0.403 D

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

When does vertex conversion matter?

Above roughly 4 dioptres, where the difference exceeds a quarter dioptre step.

Which direction does the power move?

Minus lenses become weaker at the cornea and plus lenses become stronger.