Glasses to Contact Lens Prescription Converter
Use this contact lens power calculator to estimate how a spectacle prescription may translate to contacts using vertex distance conversion. This is most helpful for moderate to high prescriptions.
Enter values in diopters. Cylinder can be negative or positive. Axis is required only when cylinder is not 0.
How this glasses prescription to contacts calculator works
This page converts a glasses prescription to an estimated contact lens prescription by accounting for vertex distance. Eyeglass lenses sit in front of your eye (often around 12 mm). Contact lenses sit directly on the eye. That position change alters effective lens power, especially when the prescription is stronger.
Where:
- F is lens power in diopters
- d is vertex distance in meters (for example, 12 mm = 0.012 m)
Why glasses and contacts are not always the same number
For lower powers (roughly within ±4.00 D), differences can be small. For higher myopia or hyperopia, conversion matters more. That is why many people notice their contact lens powers do not exactly match their eyeglass powers.
What this converter includes
- Sphere (SPH) conversion
- Cylinder/axis handling using principal meridians
- Quarter-diopter rounding, which is standard in lens ordering
- Spherical equivalent estimate
What this converter does not include
- Brand-specific fitting behavior
- Base curve (BC) and diameter (DIA) lens fit decisions
- Corneal topography or tear film factors
- Clinical over-refraction done in-office
How to read your prescription before converting
A typical eyeglass prescription has values for each eye:
- OD = right eye
- OS = left eye
- SPH = main focusing power
- CYL = astigmatism amount
- AXIS = astigmatism orientation (1 to 180)
If your CYL is zero or blank, axis is not required. If CYL has any non-zero value, axis must be present for a proper toric estimate.
Practical tips when using a contact lens prescription converter
- Use the most recent refraction from your eye exam.
- Confirm whether your prescription is written in minus-cylinder or plus-cylinder format.
- If you have high powers, rely on a professional fitting rather than a number-only conversion.
- If converted cylinder is very small, your fitter may trial a spherical contact lens first.
FAQ
Is this a valid contact lens prescription?
No. This tool provides an estimate only. A legal contact lens prescription requires a fitting and verification by a licensed eye care professional.
Can I order lenses from this result?
You should not use this result as a substitute for a prescription. Use it to understand your numbers before your appointment.
Why do I see quarter-step rounding?
Most soft contact lenses are manufactured in 0.25 D increments, so final powers are generally rounded to the nearest quarter diopter.
Bottom line
This glasses to contacts conversion tool helps you estimate contact lens powers with vertex distance math. It is useful for education and planning, but final lens selection should always be confirmed in a contact lens exam for safety, comfort, and best vision.