VertexLensCalc.
Convert Your Rx in Seconds

Contact Lens Vertex Calculator

Convert glasses to contacts or contacts to glasses using vertex distance. Enter SPH, CYL and axis to see raw theoretical powers and a 0.25 D display. This optical calculation does not issue a prescription or determine lens fit.

Calculation Steps Shown Handles Astigmatism (CYL) No Sign-up Required
Conversion direction

Enter glasses powers. The result is a theoretical contact lens equivalent at the corneal plane.

Vertex Distance ⓘ
mm
OD — Right Eye
OS — Left Eye
Enter your glasses powers above to see the theoretical conversion.
Fcl = Fspec ÷ (1 − d · Fspec)  ·  d in meters  ·  0.25 D display steps  ·  Default vertex 12 mm
HOW TO CONVERT

Convert Glasses or Contacts in Three Steps

Choose a direction, enter your prescription values and read the theoretical conversion.

  1. Choose a direction and enter your prescription

    Choose glasses to contacts or contacts to glasses. Enter Sphere (SPH) and Cylinder (CYL) from your prescription. For astigmatism, include Axis when CYL is not zero.

  2. Adjust vertex distance (default 12 mm)

    Vertex distance is the gap between your spectacle lens and cornea. Use your measured distance when available; the calculator starts at 12 mm and accepts 10–14 mm.

  3. Read your converted lens power

    View theoretical contact lens or glasses power in 0.25 D steps, alongside unrounded values. Actual lens products have their own available powers; this result does not issue a contact lens or glasses prescription.

See the conversion formulas and worked example

CONVERSION EXAMPLES

Glasses to Contacts Conversion Examples

Real prescription examples showing how sphere and cylinder power changes when converting from glasses to contact lenses — calculated at standard vertex distances of 12–14 mm.

Case OD — Right Eye OS — Left Eye Vertex
Glasses SPH / CYL Theoretical SPH Glasses SPH / CYL Theoretical SPH
Low Prescription
Mild myopia −1.00 / −0.50 -1.00 −1.25 / −0.25 -1.25 12 mm
Low hyperopia +1.50 / −0.50 +1.50 +1.25 / −0.25 +1.25 12 mm
Moderate Prescription
Moderate myopia −4.00 / −0.75 -3.75 −3.75 / −1.00 -3.50 12 mm
Astigmatism −3.00 / −1.50 -3.00 −2.75 / −1.25 -2.75 13 mm
High Prescription
High myopia −8.00 / −0.50 -7.25 −7.50 / −0.75 -6.75 14 mm
High hyperopia +6.00 / −1.00 +6.50 +5.75 / −0.75 +6.25 14 mm
Examples show theoretical sphere power only, displayed in 0.25 D steps. Cylinder is not absorbed into sphere. Use the calculator with CYL and axis for a two-meridian result; product powers and fitting are separate.
OD — Right Eye Reference OS — Left Eye Reference
Glasses SPH Theoretical SPH (12 mm) Glasses SPH Theoretical SPH (12 mm)
Minus (Myopia)
−1.00-1.00−1.00-1.00
−2.00-2.00−2.00-2.00
−3.00-3.00−3.00-3.00
−4.00-3.75−4.00-3.75
−5.00-4.75−5.00-4.75
−6.00-5.50−6.00-5.50
−8.00-7.25−8.00-7.25
−10.00-9.00−10.00-9.00
Plus (Hyperopia)
+1.00+1.00+1.00+1.00
+2.00+2.00+2.00+2.00
+3.00+3.00+3.00+3.00
+4.00+4.25+4.00+4.25
+6.00+6.50+6.00+6.50
This contact lens vertex conversion chart shows theoretical values at 12 mm for spherical inputs (CYL = 0), displayed in 0.25 D steps. For astigmatism, enter SPH, CYL and axis in the calculator.
PRESCRIPTION TERMS EXPLAINED

How to Read Your Eyeglass Prescription

To use this contact lens calculator, you need SPH and CYL from your glasses prescription. Here's what every abbreviation means — including values that appear only on contact lens prescriptions.

OD
OCULUS DEXTER

Your right eye. Always listed first on a prescription. From Latin for "right eye."

OS
OCULUS SINISTER

Your left eye. Listed second. Some modern prescriptions use LE (Left Eye) instead.

SPH
SPHERE

The main correction power. Minus (−) means myopia (nearsighted). Plus (+) means hyperopia (farsighted).

CYL
CYLINDER

Corrects astigmatism — the uneven curve of your cornea. Always paired with an Axis value. May be blank if you have no astigmatism.

AXIS
ORIENTATION

The angle (1°–180°) at which the cylinder correction is applied. Only present when CYL is not zero.

ADD
ADDITION POWER

An extra plus power for reading, used in bifocal or progressive lenses. Common in patients over 40 (presbyopia).

PD
PUPILLARY DISTANCE

The distance between your pupils in mm. Used for eyeglasses only — not needed for contact lenses.

OU
OCULUS UTERQUE

Latin for "both eyes." Used when a value — typically ADD — applies equally to both eyes.

BC
BASE CURVE

The curvature of the contact lens, matched to your cornea's shape. Contact lenses only — not on eyeglass prescriptions. Typically 8.3–8.9 mm.

DIA
DIAMETER

The total width of the contact lens in mm. Contact lenses only. Standard soft lenses are usually 14.0–14.5 mm.

D
DIOPTER

The unit of measurement for lens power. A value of −3.00 D means 3 diopters of minus correction. All SPH, CYL, and ADD values are in diopters.

To convert your glasses prescription to theoretical contact lens power, enter SPH and CYL, include Axis when CYL is not zero, and set the vertex distance (default 12 mm). Base curve (BC) and diameter (DIA) are contact-lens-only parameters — provided by your manufacturer or optometrist after a contact lens fitting exam.
FORMULAS & EXAMPLES

Both directions use the spectacle vertex distance, d, in metres. For 12 mm, enter 0.012 m in the formula.

GLASSES TO CONTACTS

How to Convert Eyeglass Prescription to Contact Lens Power

Glasses sit in front of the eye; contact lenses sit at the corneal surface. Moving the optical power between these positions requires a vertex-distance calculation.

To estimate contact lens power, use your spectacle power in the forward formula:

Forward vertex formula
Fcontacts=Fglasses1 − d × Fglasses

What changes when the lens moves?

For the powers and distances accepted by this calculator, minus spectacle powers become less negative at the corneal plane, while plus powers become more positive. The size of the change increases with power and vertex distance.

For low powers, the unrounded result can change even when the 0.25 D display stays the same.

CONTACTS TO GLASSES

How to Convert a Contact Lens Prescription to Glasses

The contacts-to-glasses calculator estimates the equivalent optical power at your chosen spectacle distance. Use the contact lens power as the input to the reverse formula:

Reverse vertex formula
Fglasses=Fcontacts1 + d × Fcontacts

Worked example

12 mm · CYL 0

−5.50 ÷ [1 + 0.012 × (−5.50)]

Contact SPH
−5.50 D
Raw glasses power
−5.8887 D
0.25 D display
−6.00 D
ASTIGMATISM

Convert both principal meridians

For spherical conversions, CYL is zero. For toric conversions, include CYL and Axis. For either direction, convert S and S + C separately. Calculate CYL from the difference between the unrounded results. S + ½C is a spherical equivalent, not the toric conversion used here.

ROUNDING

Raw power and display are different

The calculator shows raw values and a 0.25 D display. Product powers may use other increments. Reversing a rounded or product-selected value may not recover the original prescription.

Looking for ready-calculated values?View the forward and reverse vertex conversion charts
FAQ

Frequently Asked Questions

Questions about converting glasses and contact lens powers in either direction.

01

Are eyeglass and contact lens prescriptions the same?

No. Eyeglasses sit 12–14 mm from your eye; contact lenses sit directly on the cornea at 0 mm. This distance — called vertex distance — changes the effective power of the lens, so the two prescriptions differ, especially for stronger powers.

02

How do I convert my glasses prescription to contact lenses?

Enter your SPH and CYL values from your eyeglass prescription into the calculator above. Set your vertex distance (default 12 mm) and the tool applies the vertex formula Fc = Fs ÷ (1 − d × Fs) to estimate theoretical contact lens power, displayed in 0.25 D steps, with raw values also shown.

03

What formula does this calculator use?

For Glasses → Contacts, the basic vertex distance formula: Fc = Fs ÷ (1 − d × Fs), where Fs is the spectacle power and d is vertex distance in meters. For Contacts → Glasses, use Fs = Fc ÷ (1 + d × Fc). The display uses 0.25 D steps; this is a display convention, not standards certification.

04

Why is my contact lens power different from my glasses prescription?

Because the position of the lens relative to the eye changes the effective power. For −6.00 D at 12 mm, the raw theoretical power is -5.5970 D, displayed as -5.50 D in 0.25 D steps. That theoretical equivalent does not determine the final prescribed product power or fit.

05

Do daily, monthly, and toric contact lenses use the same converted power?

The same optical formula describes a change of lens plane. However, daily, monthly and toric products have different available powers and fitting parameters. The tool does not select a product or guarantee identical prescribed values across products. To budget for your prescribed lenses, use the contact lens cost calculator.

06

Does this tool handle astigmatism (CYL)?

Yes. The calculator converts S and S + C separately and rebuilds the cylinder from their difference. It retains CYL and requires axis when CYL is not zero. It does not automatically select spherical or toric lenses.

07

Can I use this result to order contacts directly?

No. This calculator gives a theoretical optical estimate. It does not issue a prescription or establish base curve, diameter, brand or fit. Confirm your prescription and fitting with your eye care professional.

08

Does vertex distance correction work for plus (hyperopia) prescriptions?

Yes. For a positive spectacle power and a positive vertex distance, the denominator 1 − d × Fspec is below 1, so the theoretical contact power is more positive. At 12 mm, +6.00 D gives +6.4655 D before rounding, displayed as +6.50 D in 0.25 D steps. This is a calculation example, not a fitted prescription.

09

What changes when converting a stronger prescription?

The theoretical change increases with the magnitude of the power and depends on vertex distance. The tool accepts SPH from −30 to +30 D and CYL from −10 to +10 D; these are input limits, not clinical suitability limits. At any power, the result does not establish a prescription, available product power or lens fit.

10

What do SPH, CYL, and Axis mean?

SPH (Sphere) is the main correction power — minus for myopia, plus for hyperopia. CYL (Cylinder) corrects astigmatism and is always paired with an Axis value (1°–180°) that indicates the orientation of the correction. Axis does not change the two meridian powers, but the tool requires it when CYL is not zero so that it never invents an orientation.

11

Does this tool calculate frame tilt or over-refraction?

No. This tool provides a basic vertex-distance conversion. It does not model front/back surface power, pantoscopic tilt, face form, over-refraction or prism. Those require different inputs and an appropriate professional assessment.

12

What do OD, OS, and OU mean on a prescription?

OD (oculus dexter) is your right eye, always listed first. OS (oculus sinister) is your left eye. OU (oculus uterque) means both eyes and is typically used for the ADD value in bifocal or progressive prescriptions. Some modern prescriptions replace OD/OS with RE/LE (Right Eye / Left Eye).

Still have questions? Use the calculator above or consult a licensed eye care professional.

Keep Exploring Contact Lens Tools
Prices, exam & fitting costs, spherical equivalent, or astigmatism conversion — all free, no sign-up.
Contact Lens Cost Calculator Estimate annual lens spending from your box prices. Exam & Fitting Cost Calculator Add quoted appointment fees and confirmed benefits. Spherical Equivalent Calculator Calculate SPH + CYL/2 as an average optical power. Toric Contact Lens Calculator Convert SPH, CYL and axis from glasses to contacts. CYL Converter Transpose plus and minus cylinder prescriptions.

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We publish educational tools and explanations about prescription power and contact lens costs. We are not an eye care practice, and no clinical review is claimed. Read our methods and limitations.

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