Vertex Distance Calculator
Convert your glasses prescription to contact lens power (or vice versa) by accounting for vertex distance.
Prescription Input
Enter values from your glasses prescription and the vertex distances.
Glasses = 12–14 mm
Contacts = 0 mm
Converted Prescription
Enter a prescription and click Calculate
| Parameter | Original | Converted |
|---|
Converted Rx (rounded to 0.25 D)
Results are for educational purposes only. Consult a licensed eye care professional before ordering lenses.
When Does Vertex Distance Matter?
Weak prescriptions (0 to ±4 D)
Vertex correction is less than 0.12 D — usually within rounding error; clinically negligible.
Moderate (±4 to ±8 D)
Correction reaches 0.25–0.75 D — one to three prescription steps. Important to account for.
High (above ±8 D)
Correction exceeds 1 D — always required; skipping it leads to a noticeably wrong contact lens power.
Summary
Convert your glasses prescription to contact lens power (or vice versa) by accounting for vertex distance.
How it works
- Enter the sphere power from your glasses prescription (e.g. −6.00).
- Enter the cylinder power if your prescription includes astigmatism (optional).
- Enter the vertex distance of your glasses — usually 12 mm; 13.5 mm or 14 mm are also common.
- Enter the target vertex distance — use 0 mm for contact lenses placed directly on the eye.
- The calculator applies the formula F₂ = F₁ / (1 − d × F₁) for each meridian and shows the adjusted powers.
Use cases
- Converting a strong glasses prescription (±4 D or higher) to contact lens power.
- Checking whether an optician correctly adjusted your prescription when ordering contacts.
- Helping eye care students understand vertex distance corrections in optics coursework.
- Comparing trial contact lens powers against a spectacle refraction.
- Calculating adjusted power when fitting lenses at a non-standard vertex distance.