Contact Lens Vertex Calculator
Convert spectacle power to contact lens power (or contact power back to glasses) at any vertex distance — with a live power curve, a chairside reference table, and full toric support for sphere, cylinder, and axis.
Reviewed for accuracy: July 2026 · Formula verified against clinical vertex-distance opticsHow the Vertex Distance Calculator Works
A spectacle lens sits about 12–14mm in front of the eye, held there by the frame. A contact lens sits directly on the cornea — effectively at a vertex distance of 0mm. Because light behaves differently depending on how far the correcting lens sits from the eye, the same optical correction needs a slightly different power at each position. This contact lens vertex calculator runs the exact effective-power formula used in clinical optics, rather than a rounded approximation, so you can see precisely how much your power shifts at any distance.
The Vertex Distance Formula
The standard back-vertex conversion is Fc = Fs ÷ (1 − d × Fs), where Fc is the corrected power at the new vertex distance, Fs is the known power, and d is the change in vertex distance in meters. Converting glasses to contacts moves the lens from 12mm to 0mm; converting a contact lens power back to glasses reverses the calculation. For toric prescriptions, the calculator applies this separately to the sphere meridian and the sphere-plus-cylinder meridian, then reconstructs the cylinder from the difference — the mathematically correct way to preserve astigmatism correction through the conversion, rather than shifting the sphere alone.
Rule of thumb: below about ±4.00D, vertex compensation is usually smaller than a single 0.25D manufacturing step and is often skipped. Above ±4.00D — and especially in high myopia, high hyperopia, or anisometropia (a large power difference between eyes) — the adjustment becomes clinically meaningful and should not be ignored.
Effective Power vs. Vertex Distance
The chart below plots how your entered power changes as vertex distance moves from the corneal plane (0mm) out to 20mm, using your current sphere values. Drag the vertex distance slider in the calculator above and watch the marked point move along the curve in real time.
Chairside Vertex Compensation Reference (at 12mm)
| Spectacle Power | Contact Lens Power (12mm) | Adjustment |
|---|
Values are rounded to the nearest 0.25D manufacturing step, consistent with standard contact lens power increments.
Spherical Equivalent for Low Astigmatism
When correcting mild, regular astigmatism with a spherical soft contact lens rather than a toric lens, practitioners often use the spherical equivalent — the sphere power plus half the cylinder — to place the circle of least confusion near the retina. This is a clinical compromise for low cylinder values, not a substitute for a toric lens when full cylindrical correction is needed. Toggle “Toric mode” above to see both the full vertex-compensated toric result and how it compares to a spherical-equivalent approach.
Common Mistakes to Avoid
- Applying vertex correction to prescriptions under ±4.00D, where the difference is smaller than an available lens increment and rarely changes the ordered power.
- Forgetting the direction reverses: for minus (myopic) prescriptions, contact lens power is usually less minus than the glasses power; for plus (hyperopic) prescriptions, it’s usually more plus.
- Ignoring significant cylinder (0.75D or higher) instead of considering a toric contact lens.
- Assuming your vertex distance is always exactly 12mm — it varies by frame fit and face shape, and your prescriber may have recorded the actual measured value.
For a full breakdown of your prescription — including reading glasses ADD power and plus/minus cylinder transposition — see our complete Eye Prescription Calculator. You can also browse the rest of the Health Calculators hub, or explore our full library of free tools from the DexoCalc homepage.
Frequently Asked Questions
A vertex distance calculator adjusts an optical lens power for the gap between the lens and the eye. It’s most often used to convert a spectacle (glasses) prescription to the equivalent contact lens power, since contacts sit directly on the cornea while glasses sit roughly 12–14mm in front of it.
Vertex distance itself is a physical measurement — typically 12 to 14mm — taken with a distometer or estimated from a trial frame during refraction. What this calculator computes is the compensated lens power for a given vertex distance, using Fc = Fs ÷ (1 − d × Fs).
Back vertex distance (BVD) is the distance from the back surface of a spectacle lens to the front of the cornea. It’s the specific measurement used in vertex compensation calculations, and is typically recorded during a comprehensive eye exam for higher prescriptions.
Generally not in practice. Below roughly ±4.00D, the vertex-compensated power differs from the original by less than a quarter diopter — smaller than the smallest manufacturing step for most contact lenses — so it’s usually not applied.
Because contact lenses sit directly on the cornea while glasses sit about 12mm in front of it, the same optical correction requires a different effective power at each position once your prescription is strong enough for the difference to matter.
The calculation is applied separately to the sphere meridian and the sphere-plus-cylinder meridian, then the cylinder is reconstructed from the difference between the two converted values. This keeps the astigmatism correction accurate rather than only adjusting the sphere component.
Spherical equivalent is the sphere power plus half the cylinder power. It’s sometimes used to fit a spherical soft contact lens over low, regular astigmatism as a clinical compromise, rather than ordering a toric lens for very mild cylinder values.
Yes — switch this calculator to “Contact Lens to Glasses Power” mode, which reverses the formula to solve for spectacle power given your contact lens power and vertex distance. A full glasses prescription also needs pupillary distance and any prism, which a contact lens power alone doesn’t capture.
12mm is the standard clinical default used during most refractions and is a reasonable assumption if your exact measurement isn’t available. If your prescriber recorded a specific back vertex distance, use that value for a more precise result.
The formula matches standard clinical optics and is precise for the sphere and cylinder power itself. However, ordering contact lenses also requires base curve, diameter, and material, which only a professional contact lens fitting can determine.
No. This tool is designed to help you understand and estimate vertex-compensated power between visits — it does not replace a comprehensive eye exam or a professional contact lens fitting.