Contact Lens Prescription Calculator
Convert your glasses prescription to contact lenses with vertex distance adjustment
Calculate Your Contact Lens Prescription
Glasses Prescription Details
Right Eye (OD)
Left Eye (OS)
Standard vertex distance is 12mm. This is the space between your glasses and your eye.
Your Converted Contact Lens Prescription
Right Eye (OD)
Left Eye (OS)
What Do These Numbers Mean?
Important Notice
This calculator provides an approximate conversion only. Contact lenses require additional measurements such as base curve (BC) and diameter (DIA) that cannot be derived from a glasses prescription. You must consult an optometrist or optician for a proper contact lens fitting and prescription before purchasing contact lenses.
About Contact Lens Prescriptions
A contact lens prescription differs significantly from a glasses prescription, even though both correct the same vision problems. The primary reason for this difference is the position of the corrective lens relative to your eye. Glasses sit approximately 12mm away from the cornea, whilst contact lenses rest directly on the surface of your eye. This distance, known as vertex distance, affects the optical power needed to correct your vision properly.
Why You Cannot Simply Transfer Prescriptions
Many people assume they can order contact lenses by transferring the numbers from their glasses prescription. However, this approach can lead to incorrect vision correction and potential eye health issues. Contact lens prescriptions include specific parameters that glasses prescriptions do not contain, such as base curve and diameter measurements. These parameters determine how the lens fits on your individual eye shape and are critical for comfort, vision quality, and eye health.
Additionally, the power correction needed changes based on the vertex distance. For moderate to high prescriptions, the difference can be as much as 0.50 to 1.00 dioptres or more. Wearing contact lenses with incorrect power can cause eye strain, headaches, blurred vision, and difficulty with daily activities.
Parameters Unique to Contact Lenses
Contact lens prescriptions include several measurements not found on glasses prescriptions:
- Base Curve (BC): Measured in millimetres (typically 8.0-9.5mm for soft lenses), this indicates the curvature of the back surface of the lens. A smaller number means a steeper curve, whilst a larger number indicates a flatter lens. The base curve must match your corneal curvature for proper fit.
- Diameter (DIA): The total width of the contact lens, usually between 13.0-14.5mm for soft lenses. This measurement affects how the lens centres on your eye and provides proper coverage.
- Brand and Material: Different manufacturers produce lenses with varying materials, water content, and oxygen permeability. Your optometrist selects the appropriate lens based on your eye health and wearing needs.
- Wearing Schedule: Whether the lenses are daily disposables, fortnightly, or monthly replacement affects the prescription parameters.
The Vertex Distance Formula
The conversion between glasses and contact lens prescriptions relies on the vertex distance compensation formula. This calculation adjusts the lens power based on the change in position from spectacles to contact lenses. The formula accounts for the fact that contact lenses sit at zero vertex distance (directly on the eye), whilst glasses typically sit at 12mm distance.
For prescriptions below ±3.50 dioptres, the vertex distance effect is minimal, and glasses and contact lens powers may be nearly identical. However, for moderate prescriptions between ±3.75 and ±5.75 dioptres, adjustments of 0.25-0.50 dioptres become necessary. For high prescriptions of ±6.00 dioptres or greater, the vertex adjustment can reach 0.75 dioptres or more, making professional calculation essential.
| Prescription Strength | Vertex Effect | Typical Adjustment |
|---|---|---|
| ±0.00 to ±3.50 D | Minimal | Usually the same power |
| ±3.75 to ±5.75 D | Moderate | 0.25-0.50 D difference |
| ±6.00 D and above | Significant | 0.75 D or more difference |
Spherical Equivalent Explained
The spherical equivalent is a simplified representation of your prescription that combines the sphere and cylinder values into a single number. This metric is particularly useful when prescribing spherical (non-toric) contact lenses for people with mild astigmatism. The calculation involves adding half of the cylinder power to the sphere power.
For example, if your glasses prescription is -3.00 sphere with -1.00 cylinder, your spherical equivalent would be -3.50 dioptres. This single number gives eye care professionals a general indication of your refractive error severity and helps determine if spherical contact lenses might provide adequate vision correction or if toric lenses (which correct astigmatism) are necessary.
Many contact lens wearers with cylinder values of -0.75 or less can achieve satisfactory vision with spherical lenses prescribed at the spherical equivalent power. However, this decision depends on individual visual needs and must be made by a qualified professional during a contact lens consultation.
Steps to Obtain Contact Lenses
Acquiring contact lenses involves several steps beyond simply converting a glasses prescription:
Initial Consultation
Your optometrist will conduct a comprehensive eye examination to assess your overall eye health and vision correction needs. This examination includes checking for conditions that might affect contact lens wear, such as dry eye syndrome, corneal irregularities, or allergies. The optometrist will also discuss your lifestyle and wearing preferences to recommend the most suitable lens type.
Contact Lens Fitting
During the fitting appointment, the optometrist measures your corneal curvature, pupil size, and other parameters specific to contact lens wear. They will select trial lenses for you to wear whilst they evaluate the fit, movement, and visual acuity. The lenses should move slightly with each blink but remain centred on your eye. Proper fitting prevents complications such as corneal abrasions, infections, or poor vision.
Training and Trial Period
First-time contact lens wearers receive instruction on proper insertion, removal, and care techniques. You will be given trial lenses to wear for several days or weeks whilst your optometrist monitors how your eyes adapt. This trial period allows for adjustments to the prescription, base curve, or lens type if needed before finalising your prescription.
Final Prescription and Supply
Once the optimal lens parameters are determined, your optometrist issues a contact lens prescription valid for a specified period (typically one or two years in the UK). This prescription includes all necessary specifications for ordering lenses. Regular follow-up appointments help monitor your eye health and prescription stability.
Frequently Asked Questions
Can I order contact lenses with just my glasses prescription?
No, you cannot legally or safely order contact lenses with only a glasses prescription. Contact lenses are classified as medical devices in the UK and require a specific contact lens prescription from an optometrist or ophthalmologist. This prescription includes measurements such as base curve and diameter that cannot be determined from glasses prescriptions alone. Attempting to order lenses without proper fitting can result in poor vision, discomfort, and serious eye complications.
How accurate is this calculator?
This calculator provides a mathematically accurate vertex distance conversion for the sphere power of your glasses prescription. However, it cannot replace a professional contact lens fitting. The converted values offer an approximation of the power you might need, but your actual contact lens prescription may differ based on factors such as tear film quality, corneal shape, and lens design. Always consult an eye care professional before purchasing contact lenses.
What if my astigmatism is high?
If you have significant astigmatism (typically cylinder values of -1.00 or greater), you will likely require toric contact lenses specifically designed to correct astigmatism. These lenses have different powers in different meridians and must be positioned correctly on your eye. Toric lens fitting is more complex than spherical lens fitting and requires precise measurements and sometimes multiple trial lenses to achieve optimal vision and comfort.
Why does vertex distance matter?
Vertex distance affects the effective power of a corrective lens. Think of it like this: the farther a magnifying glass is from an object, the more its magnification changes. Similarly, moving a corrective lens closer to or farther from your eye changes its effective power. Contact lenses sit directly on your cornea (zero vertex distance), whilst glasses sit about 12mm away. This positional difference requires a power adjustment, especially for moderate to strong prescriptions.
How often should I have my contact lens prescription checked?
In the UK, you should have your eyes examined and contact lens prescription reviewed at least every two years, or annually if you are over 40, have certain health conditions, or as recommended by your optometrist. Regular check-ups monitor not only prescription changes but also the health of your eyes and how they are responding to contact lens wear. Some complications from ill-fitting or outdated prescriptions develop gradually and can only be detected through professional examination.
Can I wear contact lenses if I have dry eyes?
Many people with dry eyes can successfully wear contact lenses with the right lens selection and care routine. Modern contact lens materials include options with high oxygen permeability and moisture retention properties specifically designed for dry eye sufferers. Your optometrist may recommend daily disposable lenses, which can be more comfortable than reusable lenses for dry eyes. They might also suggest eye drops compatible with contact lens wear or other treatments to manage dry eye symptoms.
What is the difference between daily and monthly contact lenses?
Daily disposable lenses are worn once and discarded at the end of each day, requiring no cleaning or storage. Monthly lenses are worn daily but removed each night for cleaning and storage in solution, with replacement after 30 days of use. Daily lenses offer superior convenience and hygiene but typically cost more overall. Monthly lenses are more economical but require consistent care routines. Your lifestyle, budget, and eye health considerations will help determine which option suits you best.
References
- British Standards Institution. (2018). Ophthalmic optics – Spectacle lenses – Vocabulary (BS EN ISO 13666:2019). BSI Standards Publication.
- Efron, N., Morgan, P. B., & Katsara, S. S. (2001). Validation of grading scales for contact lens complications. Ophthalmic and Physiological Optics, 21(1), 17-29.
- General Optical Council. (2023). Standards for optometrists and dispensing opticians. General Optical Council, London.
- Kollbaum, P. S., Bradley, A., & Thibos, L. N. (2008). Comparing the optical properties of soft contact lenses on and off the eye. Optometry and Vision Science, 85(6), 404-409.
- Morgan, P. B., Efron, N., Helland, M., Itoi, M., Jones, D., Nichols, J. J., van der Worp, E., & Woods, C. A. (2010). Demographics of international contact lens prescribing. Contact Lens and Anterior Eye, 33(1), 27-29.
- Rabbetts, R. B. (2007). Bennett and Rabbetts’ Clinical Visual Optics (4th ed.). Butterworth-Heinemann Elsevier.
- Royal College of Ophthalmologists. (2023). Contact Lens Prescribing Guidelines. The Royal College of Ophthalmologists, London.
- Wolffsohn, J. S., Hall, L. A., Hunt, O. A., & Banswool, K. (2010). Objective analysis of soft contact lens fit. Contact Lens and Anterior Eye, 33(Supplement 1), S43.