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Contact Lens Conversion Calculator

Glasses to contacts lens Rx and spherical equivalent

Spectacle to Contact Lens Prescription

Starting Spectacle Prescription

OD

Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type an axis from 1 to 180. Press Enter to commit. Press Escape to clear typed input.

OS

Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type an axis from 1 to 180. Press Enter to commit. Press Escape to clear typed input.

Glasses to Contact Lens Conversion: Clinical Workflow

What this calculator does

This calculator converts a glasses prescription at the trial frame or phoropter plane into a starting soft contact lens power for spherical and toric fits. It combines three steps commonly used in clinic: vertex distance compensation for higher powers, spherical equivalent (when you intentionally fit a spherical lens over low regular astigmatism), and rounding to standard manufacturing steps. The output is a starting point for ordering and documentation, followed by on-eye assessment and over-refraction. For toric troubleshooting with rotation and over-refraction, see the Cross-Cylinder Calculator.

Vertex distance and effective power

The main optical difference between glasses and contact lenses is vertex distance, typically about 12 to 14 mm between a spectacle lens and the cornea. Moving a lens from the spectacle plane to the corneal plane changes its effective power.

For minus prescriptions, effective power increases as the lens moves closer to the eye, so the ordered contact lens power is often less minus than the glasses prescription. For plus prescriptions, effective power decreases at the corneal plane, so the ordered contact lens power is often more plus.

Vertex compensation becomes clinically important once meridional powers exceed about ±4.00 D. This calculator applies vertex compensation by meridian when indicated. If you want to review vertex effects by themselves or check a borderline case, use the Vertex Distance Calculator.

Spherical equivalent in soft lens prescribing

When fitting a spherical soft lens over low, regular astigmatism, a spherical equivalent can be used to reduce blur by placing the circle of least confusion near the retina. This is a clinical compromise, not a substitute for toric correction when full cylinder correction is needed.

Spherical equivalent

SE = Sphere + (Cylinder ÷ 2)

The Contact Lens Rx result handles this for you. After any vertex compensation, if the remaining cylinder rounds to 0.50 D or less, it is folded into a spherical equivalent and a spherical lens power is returned. Otherwise it returns a toric power, with the cylinder rounded to the nearest commonly stocked toric step and the axis rounded to the nearest 10°. The separate Spherical Equivalent result is calculated at the spectacle plane, without vertex compensation. If you want to standardize plus and minus cylinder notation for ordering or documentation, use the Plus and Minus Cylinder Converter.

Suggested workflow

A common workflow for soft lens prescribing is:

  1. Enter the spectacle refraction as written, including sphere, cylinder, and axis.
  2. Allow vertex compensation for meridians beyond about ±4.00 D at the spectacle plane.
  3. Choose spherical versus toric. Use spherical equivalent only when you intentionally accept residual cylinder.
  4. Review the suggested power and round to available steps based on the manufacturer’s portfolio.
  5. Confirm fit, comfort, and over-refraction on eye, then adjust as needed.

Glasses to Contact Lens Conversion FAQs

How do I enter a glasses prescription in the calculator?

Enter the refraction exactly as written at the spectacle plane, including sphere, cylinder in minus form, and axis. The calculator applies vertex compensation when indicated and returns a spherical lens power when the remaining cylinder is 0.50 D or less. The result is a starting contact lens power that still requires on-eye evaluation, over-refraction, and lens-specific decisions such as base curve, diameter, and material.

What is spherical equivalent and when should I use it?

Spherical equivalent is calculated as SE = Sphere + (Cylinder ÷ 2).

It is most appropriate when fitting a spherical soft lens over low, regular astigmatism when some residual cylinder is acceptable. Use a toric lens when the patient is sensitive to blur, cylinder is higher, or astigmatism is irregular.

Why is contact lens power different from glasses power?

The power often differs because moving a lens from the spectacle plane to the corneal plane changes its effective power. Vertex distance is the main reason at higher prescriptions. Rounding to available steps and deciding between spherical equivalent versus toric correction can also change the final ordered power.

When should I apply vertex distance compensation?

Vertex compensation is typically relevant once spectacle powers exceed about ±4.00 D, especially when meridional powers are high. This tool applies vertex compensation when indicated. Use the Vertex Distance Calculator when you want to review the effect in isolation or teach the concept.

Do I still need separate vertex distance and cylinder tools?

This calculator covers routine glasses to contacts conversion for soft lenses, including vertex compensation when needed. Separate tools are useful when you want to isolate a single step. Use the Vertex Distance Calculator for detailed vertex analysis and the Plus and Minus Cylinder Converter to standardize notation or verify a manual transposition.

Can I use my glasses prescription to order contact lenses?

No. A glasses prescription gives lens powers at the spectacle plane, but a contact lens prescription also specifies the base curve, diameter, and lens brand or material, which come from a fitting with a trial lens on the eye. In the United States, the FTC Contact Lens Rule requires sellers to fill or verify a contact lens prescription, so a glasses prescription cannot be used to order lenses.

This calculator gives a starting contact lens power for that fitting. The final power is confirmed with over-refraction once the fit is acceptable.

Can I convert a contact lens prescription back to glasses?

Only approximately. Sphere power can be moved back to the spectacle plane with the inverse vertex formula, Fs = Fc ÷ (1 + d × Fc), where d is the vertex distance in meters. At 12 mm, a −5.50 D contact lens works back to about −5.89 D, so the original glasses sphere could have been −5.75 D or −6.00 D.

Information is also lost on the way to a contact lens. Low cylinder is often folded into a spherical equivalent, toric cylinder is rounded to stocked steps, and the axis is usually ordered in 10° steps. A refraction is the only reliable way to write a glasses prescription. This calculator converts glasses to contact lenses only.

How do you calculate the spherical equivalent?

Add half the cylinder to the sphere: SE = Sphere + (Cylinder ÷ 2), then round to the nearest 0.25 D (exact half steps round toward plus). For −2.00 −0.50 × 180, the spherical equivalent is −2.00 + (−0.25) = −2.25 D. The axis does not change the result.

This calculator shows two results. The Spherical Equivalent is calculated at the spectacle plane. The Contact Lens Rx applies vertex compensation first, so the two can differ at higher powers. For −6.00 −0.50 × 180, the spherical equivalent is −6.25 D, while the contact lens result is −5.75 D because both meridians are vertex compensated before the small remaining cylinder is folded in.

Written and reviewed by Dr. Alfonso Puzzo, Optometrist · Last reviewed