Vertex Distance Calculator | Contact Lens Power Converter
Vertex Distance Calculator
If your glasses prescription feels confusing when you try to choose contact lenses, vertex distance may be the reason. Use this tool to accurately convert your spectacle power into an estimated contact lens prescription, compensating for back vertex distance (BVD).
Calculation Result
Right Eye (OD)
Reminder: Axis is needed for a complete toric prescription, but meridional power can still be calculated.
Left Eye (OS)
Reminder: Axis is needed for a complete toric prescription, but meridional power can still be calculated.
Prescription Results
Right Eye Result
Left Eye Result
What Is Vertex Distance?
Back vertex distance (BVD) is the measurement in millimetres between the back surface of your spectacle lens and the front of your eye (the cornea). When you wear glasses, the lenses sit a small distance away from your eyes—typically around 10 to 14 mm. Contact lenses, however, sit directly on the tear film of the cornea, meaning their vertex distance is effectively 0 mm.
Because of this difference in position, the effective power of a lens changes. If you have a higher prescription (usually above ±4.00 dioptres), this change in distance is significant enough that your contact lens prescription will differ from your spectacle prescription.
How This Calculator Works
Formula Used
To convert from spectacles to contact lenses (the corneal plane), we use:
Where Fc is contact lens power, Fs is spectacle power, and d is vertex distance in metres.
Spherical vs Toric Prescriptions
If you have astigmatism, your prescription will include a cylinder (CYL) and axis. A toric calculation is more complex because the compensation must be applied to both primary meridians independently. The calculator converts the SPH to find Meridian 1, adds the CYL to find Meridian 2, applies the vertex formula to both, and then rebuilds the new SPH and CYL. The axis remains unchanged during this mathematical process.
Minus Prescriptions (Short-sighted)
For myopia, moving the lens closer to the eye increases its effective minus power. Therefore, a contact lens needs to be less minus than the glasses prescription to achieve the same visual result.
Plus Prescriptions (Long-sighted)
For hypermetropia, moving the lens closer to the eye decreases its effective plus power. Therefore, a contact lens needs to be more plus than the glasses prescription to compensate.
Vertex Distance Conversion Chart
| Spectacle (D) | Exact Contact Lens | Rounded (0.25 D) |
|---|---|---|
| -12.00 | -10.49 | -10.50 |
| -11.00 | -9.72 | -9.75 |
| -10.00 | -8.93 | -9.00 |
| -9.00 | -8.12 | -8.00 |
| -8.00 | -7.30 | -7.25 |
| -7.00 | -6.46 | -6.50 |
| -6.00 | -5.60 | -5.50 |
| -5.00 | -4.72 | -4.75 |
| -4.00 | -3.82 | -3.75 |
| +4.00 | +4.20 | +4.25 |
Common Mistakes to Avoid
- Using glasses power directly: For prescriptions over ±4.00 D, ordering contact lenses directly from a glasses prescription often results in blurry vision.
- Forgetting millimetres to metres: If doing the math manually, remember that the 12 mm BVD must be calculated as 0.012 metres.
- Vertexing only the sphere for toric lenses: You cannot simply apply the formula to the SPH and leave the CYL alone. Both meridians must be compensated.
- Treating the calculator as a final prescription: This tool provides an excellent starting estimate, but tear lens effects, lens thickness, and rotation mean an over-refraction by an eye care professional is still required.
When to Ask an Optometrist
Always consult a qualified optometrist or dispensing optician before ordering lenses, especially if:
- You have a high prescription (over ±6.00 D).
- You wear toric lenses for astigmatism.
- Your prescription does not state a specific BVD.
- You experience blurred vision, discomfort, or headaches with your current lenses.
- There is a significant prescription difference between your right and left eyes (anisometropia).
Frequently Asked Questions
Vertex distance is the space in millimetres between the back surface of your glasses lens and the front surface of your eye (the cornea). It typically ranges from 10 to 14 mm.
Because glasses sit away from your eyes and contact lenses sit directly on them, the effective optical power changes. Lenses moved closer to the eye act 'more minus' and 'less plus'.
Check your glasses prescription. In the UK, it is commonly written as 'BVD 12' or 'BVD 10'. If it is not stated, 12 mm is the standard assumption used by most optical professionals.
Vertex compensation becomes clinically significant at ±4.00 dioptres. Below this threshold, the change is usually less than a quarter dioptre (0.25 D) and does not require adjustment.
Yes, significantly. For toric lenses (used for astigmatism), both primary meridians must be calculated separately. The cylinder power will often change along with the sphere.
No. The mathematical vertex calculation does not change the axis of astigmatism. However, a contact lens might rotate when worn, which an optometrist will check during a fitting.
No. This calculator provides an accurate mathematical estimate. A final prescription requires an eye care professional to assess the physical fit, tear lens effect, and lens rotation on your eye.
Moving a minus lens closer to the eye increases its effective minus power. Therefore, you need a physically weaker (less minus) contact lens to achieve the same visual correction as your glasses.
Moving a plus lens closer to the eye decreases its effective plus power. To compensate for this loss of power, a contact lens must be made stronger (more plus) than the glasses.
It means 'Back Vertex Distance: 12 millimetres'. This indicates that the optometrist tested your vision with the trial lenses sitting exactly 12 mm away from your eyes.