👁️ Contact Lens Vertex Calculator
Switching from glasses to contacts? Convert your glasses prescription to the correct contact lens power — free and instant.
⚠️ Medical Disclaimer:
All tools on PDCalculator.net are for educational and informational purposes only. They do not replace professional eye care advice, diagnosis, or treatment. Always consult a licensed optometrist or ophthalmologist before purchasing eyewear or making any changes to your vision correction. Results are based on standard optical formulas and are provided as a general guide only.
📋 Why Glasses and Contact Lens Powers Differ
Your glasses sit 12–14mm away from your eyes. Contact lenses sit directly on the cornea (0mm distance). This changes how the lens bends light onto your retina — which means a different power is needed to achieve the same vision correction.
The difference only matters for prescriptions stronger than ±4.00D. Below that, the glasses power and contact lens power are usually the same.
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Glasses Lens
Sits 12–14mm from the cornea. Your prescription was measured at this distance.
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Contact Lens
Sits directly on the cornea at 0mm. Needs a compensated power to deliver the same vision.
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The Formula
Fc = Fs ÷ (1 − d × Fs) where d = vertex distance in meters.
❓ Frequently Asked Questions
My prescription is under ±4.00D — do I still need to convert? +
For most prescriptions under ±4.00D, the difference is small enough (less than 0.25D) that the glasses power and contact lens power are effectively the same. Above ±4.00D, the difference becomes more significant and conversion is important.
What is BVD (Back Vertex Distance)? +
BVD is the distance from the back of your glasses lens to the front of your cornea, measured in millimeters. It is usually 12–14mm. Some prescriptions include this value. If yours doesn't, use 12mm as the default.
Can I order contact lenses with just this calculation? +
No — a full contact lens prescription also includes Base Curve (BC) and Diameter (DIA), which must be measured by an eye care professional during a contact lens fitting. This tool gives you the power component only.
Do I need a doctor to use this calculator? +
No. This applies a standard optical formula. However, a full contact lens prescription from a professional is always required before purchasing contact lenses.
Why should I round to 0.25D? +
Contact lenses are manufactured in 0.25D increments. The result of the vertex calculation will often be a decimal like −3.73D. You round to the nearest 0.25D (in this case −3.75D) to match what's available in the market.