Contact Lens Vertex Distance Calculator
Contact lenses sit on the cornea, not 12–14 mm away like spectacle lenses. For prescriptions stronger than about ±4 D this distance changes the effective power needed. Enter your spectacle power and vertex distance to get the corrected contact lens prescription.
D
mm
Exact power — see rounded value below for ordering
For high prescriptions (|Vs| > 4 D) the difference between spectacle and contact lens power is clinically significant. Always confirm with an optometrist before ordering contact lenses.
- 1
Vertex distance in metres
12 ÷ 1000 = 0.012 m - 2
Denominator (1 − d × Vs)
1 − 0.012 × -5 = 1.06 - 3
Contact lens power Vc = Vs ÷ denominator
-5 ÷ 1.06 = -4.7170
How does this calculator work?
Contact lens power Vc = Vs / (1 − d × Vs) corrects for the vertex distance d (in metres) between spectacle and corneal planes. For |Vs| > 4 D the difference is clinically important — enter your spectacle power and vertex distance (typically 12–14 mm) to get the corrected contact lens power rounded to the nearest 0.25 D.
Formula
How this is calculated
A spectacle lens forms a focal point at a certain distance behind the lens. When the lens is moved closer to the eye (from the spectacle plane to the corneal plane), the focal point shifts, so a different power is needed to place it on the same point of the retina. This is the vertex distance effect, and the correcting formula is Vc = Vs / (1 − d × Vs), where Vs is the spectacle power in diopters and d is the vertex distance in metres.
For low prescriptions (−4 D to +4 D) the correction is usually less than 0.25 D and can be ignored in practice because standard contact-lens powers are supplied in 0.25 D steps. For stronger prescriptions the correction becomes significant: a −10 D spectacle prescription at a 13 mm vertex distance gives a contact lens power of about −8.85 D, a full diopter less myopia in the contact lens than the spectacles.
The result is rounded to the nearest 0.25 D to match the standard power steps in which contact lenses are manufactured. The exact (unrounded) value is also shown. Vertex distance is typically measured between 12 and 14 mm for most frames; a low-bridge frame or a large-framed wrap can push it outside this range. Always verify a contact lens prescription with a qualified optometrist — the vertex calculation is one input into a full contact lens fitting.
Frequently asked questions
Spectacle lenses sit 12–14 mm in front of the cornea. Moving the correcting lens to the corneal surface changes the effective vergence reaching the eye, so the required power changes. The higher the prescription, the larger the difference.
The difference between spectacle and contact lens power exceeds 0.25 D (the minimum step size) when the spectacle prescription is stronger than roughly ±4 D. Below that threshold the conversion is usually within manufacturing tolerance and can be ignored.
Yes. This calculator provides an estimated contact lens power from your spectacle prescription, but a contact lens fitting also considers corneal curvature (base curve), lens diameter, tear film, and the specific brand — all of which require professional assessment.
Also known as
TG we-Calculate Editorial Team. (2026). Contact Lens Vertex Distance Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/contact-lens-vertex-calculator
TG we-Calculate Editorial Team. "Contact Lens Vertex Distance Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/contact-lens-vertex-calculator.
TG we-Calculate Editorial Team, "Contact Lens Vertex Distance Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/contact-lens-vertex-calculator
@misc{wecalculate_contact_lens_vertex_calculator, title = {Contact Lens Vertex Distance Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/contact-lens-vertex-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?