Contact Lens Power Calculator

Glasses sit about 12mm from your eye (vertex distance), while contacts sit directly on the eye. This changes the effective power of the lens. The difference is significant for prescriptions above +/-4.00 D.

Vertex distance is the distance between the back surface of a spectacle lens and the front of the cornea. Standard is 12mm. Since contact lenses have zero vertex distance, a power adjustment is needed.

No. Contact lenses require a separate prescription that includes base curve, diameter, and brand, plus the power may differ from glasses. A contact lens fitting exam is legally required in most places.

For prescriptions weaker than +/-4.00 D, the difference is less than 0.25 D and usually clinically insignificant. For stronger prescriptions, the adjustment becomes increasingly important.