Vertex distance calculator

Vertex distance is the gap between the back of the spectacle lens and the front of the eye (the cornea). Use the distance actually measured for that patient and frame.

As a rule of thumb, it matters for higher powers and can be ignored for low ones. A quick test: if the converted power rounds to a different 0.25 D step than the spectacle power, the change matters. −3.00 D at 12 mm stays −3.00 D; −10.00 D becomes −9.00 D.

Fc = F ÷ (1 − d × F), where F is the spectacle power, d is the vertex distance in meters and Fc is the effective power at the cornea. For example, −10.00 D at 12 mm gives −10.00 ÷ (1 + 0.12) = −8.93 D.

Because a minus lens acts stronger as it moves closer to the eye, so the contact lens needs less minus than the glasses. A plus lens acts weaker as it moves closer, so a farsighted patient needs more plus in contacts: +10.00 D at 12 mm becomes about +11.36 D at the cornea.

Convert each principal meridian separately, then rebuild the prescription. For −6.00 −2.00 × 180 at 12 mm, the meridians −6.00 and −8.00 become about −5.60 and −7.30, which rounds to −5.50 −1.75 × 180. Do not vertex only the sphere.

Use the reverse formula: Fs = F ÷ (1 + d × F), with d in meters. For example, a −8.93 D power at the cornea is about −10.00 D in a spectacle lens worn at 12 mm.

No. It is a starting point. The final contact lens power depends on the lens design, the fit and an over-refraction, and the prescriber decides what is ordered.