Why are glasses and contact lens prescriptions different?

Glasses sit roughly 12–13 mm in front of the eye (vertex distance). Contacts sit directly on the cornea. For prescriptions stronger than about ±4.00 D, this gap changes the effective power the eye receives, so the contact lens power must be adjusted.

What vertex distance does this tool assume?

The tool uses the standard clinical vertex distance of 12 mm (0.012 m). If your optometrist measured a different value, the result will differ slightly, but 12 mm is the universal default for prescription conversion.

Can I use this result to order contacts directly?

No. This tool gives an estimate only. Contact lens prescriptions also specify base curve, diameter, brand, and material — none of which glasses prescriptions include. Always see a licensed eye care professional for a fitting and valid prescription.

What does a minus cylinder mean?

Most US prescriptions use minus cylinder notation. The cylinder value corrects astigmatism; the axis (1–180°) sets the orientation. Contacts for astigmatism are called toric lenses. This tool handles minus cylinder values.

Is cylinder converted the same way as sphere?

For contact lenses the cylinder power and axis generally carry over unchanged from the glasses prescription. Only the sphere component is adjusted for vertex distance.

What if my sphere is less than ±4.00 D?

Below ±4.00 D the vertex distance correction is clinically negligible (less than 0.25 D, which is the minimum increment lenses are made in). The converter will pass the sphere through unchanged in that range.

Contact Lens Prescription Converter

Name: Contact Lens Prescription Converter
Availability: InStock
Author: Nham Vu

Convert your glasses prescription to a contact lens prescription (or vice versa) accounting for vertex distance adjustment.

Glasses Prescription

Enter values from your glasses (spectacle) prescription. Cylinder must be in minus notation.

Vertex Distance (mm)

Standard is 12 mm. Change only if your optometrist measured a different value.

Right Eye (OD)

Sphere (SPH)

Cylinder (CYL)

Axis

Left Eye (OS)

Sphere (SPH)

Cylinder (CYL)

Axis

Estimated Contact Lens Prescription

Vertex-distance-adjusted values. For reference only — not a valid prescription.

Enter your glasses prescription and click Convert.

How the Conversion Works

Vertex Distance Formula

F_c = F_g / (1 − d × F_g)

F_c = contact power, F_g = glasses power (in diopters), d = vertex distance (in meters, typically 0.012).

When Conversion Matters

For prescriptions below ±4.00 D, the adjustment is less than 0.25 D — the smallest increment lenses are made in — so the glasses value is used unchanged.

Cylinder and Axis

Only the sphere component is adjusted for vertex distance. Cylinder power and axis carry over from the glasses prescription (toric contacts match them directly).

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Summary

Convert your glasses prescription to a contact lens prescription (or vice versa) accounting for vertex distance adjustment.

How it works

Enter your glasses prescription values: sphere (SPH), cylinder (CYL), and axis.
Choose which eye you want to convert, or run both eyes at once.
The tool applies the vertex distance formula to correct sphere power.
If the sphere power is below ±4.00 D, the correction is negligible and values carry over unchanged.
Review the estimated contact lens prescription values shown in the results panel.
Always verify the output with a licensed eye care professional before ordering lenses.

Use cases

Quickly estimate contact lens power from an existing glasses prescription.
Cross-check a prescription your optometrist provided.
Understand how vertex distance affects high-power prescriptions.
Compare contact vs. glasses prescription differences for patient education.
Help decide whether a lens trial is warranted before ordering a year supply.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu