Contact Lens Vertex Calculator
Glasses sit about 12mm in front of your eye, but contact lenses sit directly on the cornea. For prescriptions above +/-4.00 diopters, this distance change affects the effective power. A -8.00 D glasses prescription converts to about -7.41 D in contacts. This calculator does the vertex distance correction so you (or your eye care provider) can determine the correct contact lens power.
The vertex distance formula:
Fc = Fs / (1 - d × Fs)
Where Fc is the contact lens power, Fs is the spectacle power (in diopters), and d is the vertex distance in meters (typically 0.012m for glasses).
When it matters:
For prescriptions between -4.00 and +4.00 D, the difference is less than 0.25 D and can be ignored. Above that range, the correction becomes clinically significant:
| Glasses Rx | Contact Lens Power | Difference |
|---|---|---|
| -4.00 D | -3.82 D | 0.18 D |
| -6.00 D | -5.60 D | 0.40 D |
| -8.00 D | -7.27 D | 0.73 D |
| -10.00 D | -8.93 D | 1.07 D |
| +6.00 D | +6.47 D | 0.47 D |
| +10.00 D | +11.36 D | 1.36 D |
Key patterns:
- Minus lenses: The contact lens power is always less minus than the spectacle power. The contacts are weaker because they are closer to the eye.
- Plus lenses: The contact lens power is always more plus than the spectacle power. The contacts need to be stronger.
- The higher the Rx, the bigger the difference. At -10.00 D, skipping vertex correction means being over a full diopter off.
Important: This only converts the sphere component. If your prescription has cylinder (astigmatism), your eye care provider needs to evaluate whether toric contacts, spherical contacts, or rigid gas permeable lenses are the best option.