Contact Lens Vertex Calculator

Convert your spectacle prescription to the correct contact lens power using vertex distance compensation. Enter your sphere, cylinder, axis, and vertex distance — and get the compensated contact lens sphere, cylinder, and final power, rounded to the nearest 0.25 D for practical ordering.

D

Enter the sphere power from your glasses prescription (negative for myopia, positive for hyperopia).

D

Enter the cylinder power. Leave as 0 if you have no astigmatism.

°

Axis in degrees (1–180). Only needed if cylinder is non-zero.

mm

The distance between the back of your spectacle lens and your cornea. Typically 12–14 mm.

Choose toric to correct astigmatism, or spherical equivalent if fitting a spherical lens over low astigmatism.

Results

Contact Lens Sphere

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Contact Lens Cylinder

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Axis

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Spherical Equivalent

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Compensated Sphere (unrounded)

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Compensated Cylinder (unrounded)

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Spectacle vs Contact Lens Power Comparison

Frequently Asked Questions

What is BVD 12 on an eye prescription?

BVD stands for Back Vertex Distance — the distance in millimetres between the back surface of your spectacle lens and the front of your cornea. A BVD of 12 means 12 mm, which is a common standard value used in clinics. It matters because the effective power of a lens changes as it moves closer to or farther from the eye, which is why vertex compensation is needed when switching from glasses to contact lenses.

What is the vertex of a contact lens?

The vertex of a contact lens refers to the concept of vertex distance — the separation between a corrective lens and the eye. Because contact lenses sit directly on the cornea, their vertex distance is effectively zero. Spectacle lenses, sitting ~12–14 mm away, need their power adjusted (vertex compensated) to provide the same effective correction when prescribed as contact lenses.

Why can't I just use the same power for contacts as my glasses?

The effective power of a lens depends on its distance from the eye. At higher prescriptions (typically beyond ±4.00 D), even a few millimetres of difference causes a clinically significant change in perceived power. A spectacle lens sitting 12 mm away behaves differently than a contact sitting on the cornea, so the powers must be mathematically compensated using the vertex distance formula.

How do you calculate vertex compensation for toric lenses?

For toric lenses, vertex compensation is applied to the sphere and cylinder components separately. The formula Fc = Fo / (1 − d × Fo) is applied to the sphere power and again to the sphere-plus-cylinder power. The resulting compensated cylinder is the difference between these two compensated values. The axis remains unchanged. Both results are then rounded to the nearest 0.25 D for standard manufacturing steps.

What is a spherical equivalent and when is it used?

The spherical equivalent (SE) is calculated as: SE = Sphere + (Cylinder / 2). It is used when fitting a spherical contact lens for a patient with low astigmatism, where the practitioner intentionally ignores the cylinder correction. The SE gives a single sphere power that approximates the overall refractive error without specifying a cylinder or axis.

What happens when a plus lens is moved closer to the eye?

When a plus (positive) lens is moved closer to the eye, its effective power decreases. This means a patient wearing high plus spectacle lenses will need a slightly lower plus power in their contact lenses. Conversely, moving a minus lens closer to the eye increases its effective power, so a high minus contact lens prescription is slightly stronger than the equivalent spectacle prescription.

What lens power compensates a +8.0 D spectacle lens at 12 mm vertex distance?

Using the formula Fc = Fo / (1 − d × Fo), where d = 0.012 m and Fo = +8.0 D: Fc = 8.0 / (1 − 0.012 × 8.0) = 8.0 / 0.904 ≈ +8.85 D. Rounded to the nearest 0.25 D, the contact lens power would be +8.75 D. This calculator performs this computation automatically for both sphere and cylinder components.

At what prescription power does vertex compensation become important?

Vertex compensation is generally considered clinically significant when the spectacle prescription is greater than ±4.00 D in sphere or cylinder. Below this threshold, the difference between the spectacle and contact lens power is typically less than 0.25 D — within the standard rounding step — and may not require adjustment. Above ±4.00 D, compensation is essential for accurate fitting.

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