Contact Lens Vertex Calculator
Enter a glasses lens power and the vertex distance to convert it to the equivalent contact lens power.
Educational conversion — not a prescription. This tool performs the optical vertex conversion for education only. It is not a prescription and does not replace an eye exam. Contact lenses require a fitting and a valid prescription from a licensed eye care professional.
Because a contact lens sits on the eye while glasses sit about 12 mm in front of it, strong prescriptions need a vertex adjustment. The contact power equals the spectacle power divided by (1 minus the vertex distance in metres times the spectacle power). The correction matters most above roughly ±4 diopters.
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How to Use This Calculator
Enter the spectacle lens power in diopters — negative for nearsightedness, positive for farsightedness — and the vertex distance, which is how far the glasses sit from the eye, about 12 mm on average. The calculator returns the equivalent contact lens power, rounded to the nearest quarter diopter as lenses are made.
The adjustment is small for weak prescriptions but becomes significant for strong ones, which is why it matters mainly above about ±4 diopters.
How the Calculation Works
Lens power depends on the distance from the eye. Moving a lens from the spectacle plane onto the cornea changes its effective power, and the vertex formula corrects for that shift.
The spectacle power is divided by one minus the product of the vertex distance in metres and the spectacle power. For minus lenses the contact power comes out weaker; for plus lenses it comes out stronger. The result is rounded to the nearest 0.25 D, the standard manufacturing step.
F_contact = F_glasses ÷ (1 − d × F_glasses), d in metres
Worked Example
For a −8.00 D glasses prescription at a 12 mm vertex distance: d = 0.012 m, so the contact power is −8 ÷ (1 − 0.012 × −8) = −8 ÷ (1 + 0.096) = −8 ÷ 1.096 = −7.30 D.
Rounded to the nearest 0.25, that is −7.25 D — noticeably weaker than the glasses, which is why high prescriptions must be converted.
Assumptions and Limitations
- Converts sphere power only, not cylinder or axis for astigmatism
- Uses an average 12 mm vertex distance unless you change it
- The adjustment is negligible below about ±4 D
- Not a prescription — contacts require a professional fitting
When to Use an Eye Prescription Calculator
Glasses wearers considering contact lenses use this to convert a spectacle prescription to contact-lens power using the vertex adjustment, which matters most at stronger powers. It helps you understand your numbers before an eye exam. It is educational only — an eye-care professional must confirm any contact-lens prescription and fit.
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Open calculator →Frequently Asked Questions
Why are contact lens and glasses prescriptions different?
Because glasses sit about 12 mm in front of the eye and contacts sit directly on it. That distance changes the lens's effective power, so strong prescriptions need a vertex conversion.
At what power does vertex conversion matter?
The correction is small below about ±4 diopters and becomes clinically important above it. High myopes and hyperopes see the biggest difference between their glasses and contact powers.
What is vertex distance?
It is the gap between the back of a spectacle lens and the front of the eye, averaging around 12 mm. It is the distance the vertex formula uses to adjust the power.
Can I use this to order contacts?
No. It shows the optical conversion for learning. Contact lenses require a fitting for base curve, diameter and comfort, plus a valid prescription from an eye care professional.
Does this handle astigmatism?
It converts the sphere power only. Astigmatism (cylinder and axis) and toric contact fitting are more complex and must be handled by your eye care professional.
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