Vertex Distance Calculator for Contact Lenses & Glasses
Why glasses and contact lens powers differ
If you have ever tried to order contact lenses using your glasses prescription, you may have noticed that the numbers do not always match. This occurs because of back vertex distance (BVD) —the physical distance between the back of your spectacle lens and the front of your eye.
When a lens moves closer to the eye (such as a contact lens that sits directly on the cornea), its effective optical power changes. Our vertex distance calculator instantly converts your glasses prescription into a practical starting power for contact lenses.
How the vertex distance calculator works
This tool uses the standard optical formula to adjust your prescription. You simply enter your sphere (and cylinder/axis if you have astigmatism), confirm the vertex distance (usually 12 mm in the UK), and the calculator provides both the exact mathematical result and the nearest 0.25 D practical starting point.
When vertex distance matters
Vertex compensation is generally only necessary for prescriptions of ±4.00 Dioptres or higher. For weaker prescriptions, the change in power is so small that it is clinically insignificant, meaning your glasses and contact lens powers will be identical.
Glasses vs contact lens power
As a general rule of thumb:
- Short-sighted (Myopia): You will need less minus power in contact lenses than in glasses.
- Long-sighted (Hyperopia): You will need more plus power in contact lenses than in glasses.
Toric and astigmatism calculations
For patients with astigmatism, simply adjusting the sphere power is not sufficient. The calculator applies vertex compensation to both principal meridians separately, ensuring the cylinder power is accurately adjusted for the corneal plane.
Vertex distance calculation formula
The effective power of a lens changes depending on the distance it is moved. The formula used is:
Where:
- Fc = Compensated contact lens power (corneal plane)
- Fs = Spectacle lens power
- d = Vertex distance in metres (e.g., 12 mm = 0.012 m)
Quick 12 mm conversion chart
For quick reference, here is a conversion chart for common spherical powers at a standard 12 mm vertex distance, which will be very helpful.
| Glasses (Spectacle) Power | Contact Lens Power (12 mm) |
|---|---|
| -4.00 D | -3.75 D |
| -5.00 D | -4.75 D |
| -6.00 D | -5.50 D |
| -7.00 D | -6.50 D |
| -8.00 D | -7.25 D |
| -9.00 D | -8.00 D |
| -10.00 D | -8.75 D |
| +4.00 D | +4.25 D |
| +5.00 D | +5.25 D |
| +6.00 D | +6.50 D |
| +7.00 D | +7.75 D |
| +8.00 D | +8.75 D |
Frequently Asked Questions (FAQs)
What is a vertex distance calculator?
It is a tool used by optometrists, dispensing opticians, and patients to convert a glasses prescription into an accurate contact lens prescription by accounting for the distance between the lens and the eye.
What is back vertex distance?
Back vertex distance (BVD) is the measurement in millimetres from the back surface of a spectacle lens to the front surface of the cornea. The standard default is usually 12 mm.
Does vertex distance matter for astigmatism / toric lenses?
Yes. If the sphere or cylinder power is high enough to require adjustment, the astigmatism correction must also be vertexed. Our calculator adjusts both principal meridians accurately for toric lenses.
Is this result exact or a starting point?
The rounded result is a highly accurate starting point. However, contact lens fitting also involves assessing how the lens moves and settles on the eye, so an over-refraction by an eye care professional is always recommended to finalise the prescription.