Estimate contact lens power from a glasses prescription using vertex distance correction for spherical or toric prescriptions.
Nominal rounded estimate: −6.50 −1.50 at axis 180 degrees.
Convert one eye at a time from spectacle-plane power to an estimated corneal-plane starting power.
Choose plus- or minus-cylinder notation to match the glasses prescription.
Enter sphere, cylinder magnitude, and axis for one eye.
Enter the measured back vertex distance in millimeters.
Review the exact principal-meridian conversion and nominal rounded result.
For an astigmatic prescription, sphere and sphere-plus-cylinder form two different principal powers. Each power responds differently to a change in lens position.
Signed spectacle power is moved from the glasses plane to the corneal plane using distance in meters.
For a detailed optical treatment, see the National Academy of Opticianry reference on vertex distance and compensated power .
Corneal-plane power
FCL = Fspec / (1 − dFspec)
d = back vertex distance in meters
Divide millimeters by 1,000 to obtain meters.
Apply the signed formula to sphere and sphere-plus-cylinder independently.
Find the cylinder from the converted difference, then round each meridian nominally.
For −7.00 −1.75 × 180 at 12.0 mm, the principal meridians are −7.00 D and −8.75 D. Vertexing them separately gives approximately −6.46 D and −7.92 D.
Power conversion cannot select lens material, base curve, diameter, brand, fit, rotation allowance, or over-refraction. Contact lenses require an examination and fitting by a licensed eye-care professional.
The calculation uses signed dioptric power, independent principal-meridian conversion, and nominal quarter-diopter rounding.
The result is an educational corneal-plane estimate and does not claim clinical review or produce a valid contact lens prescription.
Practical answers about spectacle-to-contact-lens power conversion.
Vertex compensation becomes more important as lens power and distance increase. Many clinical references emphasize it around powers of ±4.00 D and above, but this calculator shows the continuous mathematical change at every power.
A high-minus spectacle lens moved closer to the eye needs less minus power to produce similar vergence at the cornea. High-plus powers move in the opposite numeric direction and become more plus.
No. Vertex conversion changes the powers of the two principal meridians, not their orientations. Axis changes by 90° only when the same prescription is transposed between plus- and minus-cylinder notation.
The calculator rounds each principal meridian independently to the nearest 0.25 D, with exact half steps rounded away from zero. These values are mathematical nominal increments and are not product-specific availability recommendations.
Manufactured contact lenses come in product-specific sphere, cylinder, and axis steps. Tear-lens effects, rotation, over-refraction, lens design, and fit can also change the final prescribed parameters.
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