Right Eye (OD)
D
Sphere power (+ or −)
Enter sphereD
mm
Standard = 12–14mm
Left Eye (OS)
D
Sphere power (+ or −)
Enter sphereD
→
Vertex correction applies to prescriptions stronger than ±4.00D
Contact Lens Prescription
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What Is Vertex Distance and Why Does It Matter?
Vertex distance is the space between the back surface of a spectacle lens and the front of your eye, typically 12–14mm. When you wear glasses, the lens sits at this distance from your cornea. Contact lenses, however, sit directly on the cornea at 0mm vertex distance. This difference means a high prescription in glasses doesn't translate directly to contacts — the power must be mathematically converted.
Contact Lens Power = F_glasses ÷ (1 − d × F_glasses)
Where: F_glasses = spectacle Rx in diopters · d = vertex distance in meters (e.g., 0.012 for 12mm)
Example: −8.00D glasses, 12mm vertex:
CL Power = −8.00 ÷ (1 − 0.012 × −8.00) = −8.00 ÷ 1.096 = −7.25D
Example: −8.00D glasses, 12mm vertex:
CL Power = −8.00 ÷ (1 − 0.012 × −8.00) = −8.00 ÷ 1.096 = −7.25D
💡 When vertex conversion matters: For prescriptions of ±4.00D or less, the vertex difference is usually small enough to be clinically insignificant (less than 0.25D). For prescriptions stronger than ±4.00D, the conversion can change your contact lens power by 0.25D–1.00D or more.
⚠️ Always confirm with your eye care provider. This calculator provides a mathematical estimate. Only a licensed optometrist or ophthalmologist can fit and prescribe contact lenses, which also require a contact lens exam that includes corneal curvature (base curve) measurements.
Frequently Asked Questions
What is vertex distance in contact lens fitting?
Vertex distance is the measurement from the back surface of a spectacle lens to the front of the cornea, typically 12–14mm for standard eyeglass frames. Because contact lenses sit directly on the cornea (zero vertex distance), high prescriptions must be converted using the vertex distance formula. The difference in power becomes clinically significant for prescriptions greater than ±4.00 diopters.
How do I convert my glasses prescription to contacts?
Use the vertex distance formula: CL Power = F ÷ (1 − d × F), where F is your spectacle sphere power in diopters and d is the vertex distance in meters. For example, a −6.50D spectacle prescription at 12mm vertex = −6.50 ÷ (1 + 0.012 × 6.50) = −6.50 ÷ 1.078 = −6.03D, rounded to −6.00D. This calculator performs the conversion automatically for both eyes.
Do I need a vertex calculation for a low prescription?
For prescriptions of ±4.00D or less, the vertex correction is typically less than 0.25D — smaller than the standard contact lens power increment — so it is usually clinically insignificant. Most eye care professionals do not apply a vertex conversion for low prescriptions. For prescriptions above ±4.00D, vertex correction becomes important and can change the final contact lens power by 0.25D to 1.00D.
Can I use my glasses prescription directly for contacts?
For low prescriptions (under ±4.00D), glasses and contact lens sphere powers are often the same or differ by only 0.25D. For higher prescriptions, you need a vertex-adjusted power. Additionally, contact lens prescriptions include additional parameters not in glasses prescriptions: base curve (BC) and diameter (DIA), measured during a contact lens fitting exam. A glasses prescription alone is not sufficient to order contact lenses.
What is a standard vertex distance for glasses?
The standard vertex distance for spectacle lens prescriptions is 12mm to 14mm, with 12mm being the most common default. Your optometrist may record the specific vertex distance used during your refraction on your prescription or lens order. High-power prescriptions are especially sensitive to vertex distance — even a 2mm change can alter the effective power by 0.25D or more at high prescriptions.
What is the difference between OD and OS in prescriptions?
OD stands for "Oculus Dexter" — the right eye. OS stands for "Oculus Sinister" — the left eye. OU stands for "Oculus Uterque" — both eyes. These Latin abbreviations are universally used in optometry and ophthalmology prescriptions worldwide. Some modern prescriptions use "RE" (right eye) and "LE" (left eye) instead, but OD/OS remains the professional standard.
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