Eyeglass to Contact Lens Power Conversion Chart — Complete Scientific Guide
Introduction
Many eyeglass wearers wonder: can I use the same prescription for contact lenses? The scientific answer: not always. There is a fundamental difference between the two prescriptions that must be understood before choosing your lenses.
Why Does an Eyeglass Prescription Differ from a Contact Lens Prescription?
Eyeglasses sit approximately 12–14 mm away from the cornea — a measurement known as the Vertex Distance — while contact lenses rest directly on the cornea. This difference in distance changes the effective optical power that reaches the eye.
Scientific Rule: For prescriptions exceeding ±4.00 diopters, the difference between the two prescriptions becomes significant and must be adjusted.
The Scientific Conversion Formula
Use this formula to convert your eyeglass power to contact lens power:
Contact Lens Power = Eyeglass Power ÷ (1 − 0.012 × Eyeglass Power)
Where 0.012 represents the vertex distance in meters (12 mm).
Eyeglass to Contact Lens Conversion Chart
The table below shows common prescription conversions. For prescriptions below ±4.00, the values are typically the same.
Important Notes About Your Prescription
- Astigmatism (CYL): If you have a cylindrical prescription, you will need specialized Toric contact lenses.
- PD Measurement: Not used in contact lenses — they automatically center on the cornea.
- Low Prescriptions (less than ±4.00): Usually require no adjustment between eyeglass and contact lens power.
Frequently Asked Questions
Can I order contact lenses with my eyeglass prescription?
Not directly. Contact lens prescriptions require additional measurements such as base curve (BC) and diameter (DIA), which are not included in an eyeglass prescription.
What is vertex distance and why does it matter?
Vertex distance is the space between the back of your eyeglass lens and the front of your eye. Because contact lenses sit directly on the eye, the effective power changes — especially for prescriptions above ±4.00 diopters.
Do I need Toric lenses for astigmatism?
Yes. Standard spherical contact lenses cannot correct astigmatism. Toric lenses are specially designed to address cylindrical (CYL) prescriptions.
When Should You Consult an Optician?
- For high prescriptions (more than ±4.00 diopters)
- When astigmatism is present
- With any sudden change in vision
- When wearing contact lenses for the first time
Shop Contact Lenses at Optic Store
At Optic Store, we offer a wide range of contact lenses for all prescriptions — including Toric lenses for astigmatism — with free expert consultation to help you find the perfect lens for your eyes.