Contact Lens Prescription Converter
Use this calculator to estimate your contact lens prescription based on your current glasses prescription. Please note that this is an approximation, and a professional eye care practitioner must always perform a contact lens fitting and provide the final prescription.
Understanding Contact Lens Prescription Conversion
When you get a prescription for eyeglasses, the lenses sit a small distance away from your eyes. This distance is called the "vertex distance." For contact lenses, the lens sits directly on the surface of your eye. This difference in distance means that a direct conversion of your glasses prescription to contact lenses isn't always accurate, especially for higher prescriptions.
Why is Conversion Necessary?
The effective power of a lens changes with its distance from the eye. For prescriptions with a sphere power (SPH) greater than approximately ±4.00 diopters, this difference becomes significant enough to require adjustment. Without conversion, a strong glasses prescription might feel too strong or too weak as a contact lens, leading to blurry vision or discomfort.
The Conversion Formula
The most common formula used for vertex distance correction is:
Pc = Pg / (1 - d * Pg)
Pcis the corrected contact lens power.Pgis the glasses prescription power (sphere).dis the vertex distance in meters (e.g., 12mm = 0.012m).
For example, if your glasses prescription is -6.00D and your vertex distance is 12mm (0.012m):
Pc = -6.00 / (1 - 0.012 * -6.00)
Pc = -6.00 / (1 + 0.072)
Pc = -6.00 / 1.072 ≈ -5.60D
This would typically be rounded to the nearest available contact lens power, often in 0.25D increments (e.g., -5.50D or -5.75D).
Cylinder and Axis
For astigmatism, the cylinder (CYL) and axis (AXIS) values from your glasses prescription are often directly transferable to contact lenses. However, contact lenses for astigmatism (toric lenses) are not available in all cylinder powers or axes. Your eye care professional will determine the best available match during a fitting.
Important Considerations
- Professional Fitting is Crucial: This calculator provides an estimate. A comprehensive eye exam and contact lens fitting by an optometrist or ophthalmologist are essential. They will assess your eye health, measure your eye's curvature, and determine the best lens type, brand, and exact power for your individual needs.
- Lens Availability: Contact lens manufacturers produce lenses in specific increments of sphere, cylinder, and axis. Your final prescription will be the closest available power that provides optimal vision and comfort.
- Comfort and Vision: Even with the correct power, contact lenses must fit properly and be comfortable. A poorly fitting lens can cause irritation, blurry vision, or even damage to your eye.
- Vertex Distance Variation: While 12mm is a common average, individual vertex distances can vary. If you know your specific vertex distance, using that value will yield a more accurate estimate.
Examples of Conversion
- Example 1: Moderate Nearsightedness
Glasses SPH: -3.00D, CYL: -0.75D, AXIS: 180
Vertex Distance: 12mm
Result: Since -3.00D is less than 4.00D (absolute value), the sphere power typically remains -3.00D. CYL and AXIS would likely be -0.75D and 180 respectively, subject to availability. - Example 2: Strong Nearsightedness
Glasses SPH: -7.50D, CYL: -1.25D, AXIS: 90
Vertex Distance: 12mm
Calculation: Pc = -7.50 / (1 – 0.012 * -7.50) = -7.50 / (1 + 0.09) = -7.50 / 1.09 ≈ -6.88D. Rounded to nearest 0.25D, this might be -6.75D or -7.00D. CYL and AXIS would likely be -1.25D and 90. - Example 3: Strong Farsightedness
Glasses SPH: +5.50D, CYL: None, AXIS: None
Vertex Distance: 14mm
Calculation: Pc = +5.50 / (1 – 0.014 * +5.50) = +5.50 / (1 – 0.077) = +5.50 / 0.923 ≈ +5.96D. Rounded to nearest 0.25D, this might be +6.00D.
Always consult with your eye care professional for your final contact lens prescription and fitting.