Eyewear and Vision
Vertex distance conversion.
Enter a glasses prescription and a vertex distance to see the equivalent power at the surface of the eye, or go the other way. It converts both principal meridians. Runs in your browser; nothing is sent or stored.
Prescription and vertex distance
Converted power
Fill the form and press Calculate.
The short answer
Contact lens power = 1 / (1/F - d), with F the glasses power in dioptres and d the vertex distance in metres. At 12 mm, -8.00 D in glasses is about -7.30 D at the eye. It matters mainly beyond +/-4.00 D.
Worked example
Wikipedia's worked case: -8.00 -5.25 x 085 measured at 12 mm. The two principal meridians are -8.00 D and -13.25 D. Converted to the eye: 1 / (1/-8 - 0.012) = -7.30 and 1 / (1/-13.25 - 0.012) = -11.43, so the cylinder is -4.13. The calculator shows -7.25 -4.25 x 085, with the unrounded -7.30 -4.13 x 085 underneath.
Going the other way, a -8.00 D contact lens corresponds to 1 / (1/-8 + 0.012) = -8.85 in glasses.
How it works
Moving a lens closer to the eye changes the power that is needed. For power F in dioptres and a vertex distance x in metres, glasses to contacts: Fc = 1 / (1/F - x); contacts to glasses: F = 1 / (1/Fc + x). The calculation is done on each principal meridian (the sphere power, and the sphere plus the cylinder), and the new cylinder is the difference. The axis does not change.
The change is small below about 4 dioptres, which is why Wikipedia describes vertex correction as significant when a prescription is beyond +/-4.00 D. Short-sighted (minus) powers need a weaker contact lens than glasses; long-sighted (plus) powers need a stronger one. The calculator rounds the result to the nearest 0.25 D, the usual lens step, as a convenience only.
Important. A contact lens prescription also needs base curve, diameter and a fitting. This tool only converts the power. Do not buy contact lenses from a converted number; get a contact lens fitting and prescription.
Sources (opened 2026-10-08): Wikipedia, Vertex distance.
FAQ
How do I convert a glasses prescription to contact lens power?
Use Fc = 1 / (1/F - x), where F is the glasses power in dioptres and x is the vertex distance in metres. At 12 mm, a -8.00 D glasses power is about -7.30 D at the eye. The calculator applies it to sphere and cylinder meridians.
What vertex distance should I use?
A commonly used value is 12 mm, as in the worked example on Wikipedia. Your own measured value may differ, and an eye-care professional can measure it.
When does vertex distance matter?
Mostly when the prescription is stronger than about +/-4.00 D. Below that, the converted power is within a quarter dioptre of the original.
Do I need to change the axis?
No. The axis stays the same. Only the sphere and cylinder powers change.
Is this a contact lens prescription?
No. Contact lenses also need base curve, diameter and a fitting. This converts power only and must not be used to order lenses.
Why is a plus power stronger in contacts?
Moving a plus lens closer to the eye shifts its focus further back, so a stronger lens is needed to compensate. A minus lens works the other way and gets weaker.
Disclaimer
Not medical advice. This tool does simple arithmetic on numbers you type in. It does not replace an eye exam, a prescription or the advice of an optometrist or ophthalmologist, and it is not a way to order or change a prescription. Contact lenses need a professional fitting.