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.
📋 When Do You Need Reverse Vertex?
You have your contact lens prescription but want to order glasses — but the powers aren't the same! Because glasses sit further from your eye than contact lenses, you need a compensated power. This tool does the reverse of the standard vertex calculation to find the equivalent spectacle power.
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Contact Lens
Power measured at 0mm from the cornea (rests directly on the eye).
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Glasses
Lens sits 12–14mm from the cornea. A different power is needed for the same vision.
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Reverse Formula
Fs = Fc ÷ (1 + d × Fc) where d = target vertex distance in meters.
❓ Frequently Asked Questions
Is my glasses power exactly the same as my contact lens power? +
Usually not, especially for stronger prescriptions. For powers under ±4.00D the difference is less than 0.25D and often the same. For higher powers, vertex compensation becomes important and the glasses power will be noticeably different from the contact lens power.
Why does the formula use the vertex distance in meters? +
The vertex compensation formula requires consistent units. Lens power is measured in diopters (1/meters), so the distance must also be in meters. 12mm becomes 0.012m in the formula.
Can I use this result to order glasses online? +
This tool gives you the power component only. A full glasses prescription also includes PD (pupillary distance) and possibly prism. Use our PD Calculator on this site to measure your PD, then combine with this result.
My contact lens prescription says BC and DIA — what do I do? +
BC (Base Curve) and DIA (Diameter) are contact lens fitting parameters and have no equivalent in a glasses prescription. You only need the sphere (SPH) and cylinder (CYL) values for this conversion.
Do I need a doctor to use this? +
No. This applies a standard optical formula. However, always verify with a licensed optometrist before ordering glasses to ensure accuracy and that no other prescription elements are missing.