Eyeglass to Contact Lens Prescription Calculator
Convert your glasses prescription to contact lens power in seconds. Uses the clinical vertex distance formula — the same standard optometrists use to calculate sphere and cylinder correction for daily, monthly, and toric lenses.
Three Steps to Convert Your Glasses Prescription
Enter your SPH and CYL values, set your vertex distance, and get your contact lens power — calculated using the same clinical formula optometrists use.
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Enter your glasses prescription (SPH & CYL)
Enter the Sphere (SPH) and Cylinder (CYL) values from your glasses prescription. If you have astigmatism, include the CYL value — the calculator handles both spherical and toric conversions.
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Adjust vertex distance (default 12 mm)
Vertex distance is the gap between your spectacle lens and cornea. The ISO 8980-1 default is 12 mm — change it only if your optometrist noted a different value on your prescription.
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Get your converted contact lens power
Your contact lens prescription is calculated using the vertex distance formula and rounded to the nearest 0.25 D per ANSI Z80.1 — the same rounding standard used in clinical practice.
Glasses to Contact Lens 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 | Contact Lens SPH | Glasses SPH / CYL | Contact Lens SPH | ||
| Low Prescription | |||||
| Mild myopia | −1.00 / −0.50 | −1.25 | −1.25 / −0.25 | −1.25 | 12 mm |
| Low hyperopia | +1.50 / −0.50 | +1.25 | +1.25 / −0.25 | +1.00 | 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 | −2.75 | −2.75 / −1.25 | −2.50 | 13 mm |
| High Prescription | |||||
| High myopia | −8.00 / −0.50 | −7.50 | −7.50 / −0.75 | −7.00 | 14 mm |
| High hyperopia | +6.00 / −1.00 | +5.75 | +5.75 / −0.75 | +5.50 | 14 mm |
| OD — Right Eye Reference | OS — Left Eye Reference | |||
|---|---|---|---|---|
| Glasses SPH | Contact Lens SPH (12 mm) | Glasses SPH | Contact Lens SPH (12 mm) | |
| Minus (Myopia) | ||||
| −1.00 | −1.00 | −1.00 | −1.00 | |
| −2.00 | −2.00 | −2.00 | −2.00 | |
| −3.00 | −2.75 | −3.00 | −2.75 | |
| −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 | −7.50 | −8.00 | −7.50 | |
| −10.00 | −9.25 | −10.00 | −9.25 | |
| Plus (Hyperopia) | ||||
| +1.00 | +1.00 | +1.00 | +1.00 | |
| +2.00 | +1.75 | +2.00 | +1.75 | |
| +3.00 | +2.75 | +3.00 | +2.75 | |
| +4.00 | +3.75 | +4.00 | +3.75 | |
| +6.00 | +5.75 | +6.00 | +5.75 | |
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.
Your right eye. Always listed first on a prescription. From Latin for "right eye."
Your left eye. Listed second. Some modern prescriptions use LE (Left Eye) instead.
The main correction power. Minus (−) means myopia (nearsighted). Plus (+) means hyperopia (farsighted).
Corrects astigmatism — the uneven curve of your cornea. Always paired with an Axis value. May be blank if you have no astigmatism.
The angle (1°–180°) at which the cylinder correction is applied. Only present when CYL is not zero.
An extra plus power for reading, used in bifocal or progressive lenses. Common in patients over 40 (presbyopia).
The distance between your pupils in mm. Used for eyeglasses only — not needed for contact lenses.
Latin for "both eyes." Used when a value — typically ADD — applies equally to both eyes.
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.
The total width of the contact lens in mm. Contact lenses only. Standard soft lenses are usually 14.0–14.5 mm.
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.
How to Convert Eyeglass Prescription to Contact Lens Power
Your glasses prescription and contact lens prescription are not interchangeable — even when your vision hasn't changed. The reason is vertex distance: spectacle lenses sit approximately 12 mm in front of the cornea, while contact lenses rest directly on the corneal surface at 0 mm. That gap alters the effective optical power your eye actually receives.
For prescriptions below ±4.00 D, the difference is small enough that opticians sometimes skip the correction. But for moderate to high prescriptions, applying glasses values directly to contact lenses causes measurable overcorrection or undercorrection — leading to blurred vision, eye strain, and headaches that are entirely preventable with a proper vertex distance conversion.
Vertex Distance Formula
Fcl = Fspec ÷ (1 − d × Fspec)
For astigmatic corrections, the formula is applied to both the sphere value and the effective sphere (sphere + ½ cylinder) separately before rounding.
Why vertex distance matters above ±4.00 D
At −6.00 D, the spectacle lens delivers approximately −5.75 D at the corneal plane. That 0.25 D difference is clinically significant — enough to cause persistent blur, headaches, and eye fatigue in daily contact lens wear.
Glasses vs contact lens prescription: key differences
Contact lens prescriptions include base curve (BC) and diameter (DIA) — parameters glasses prescriptions don't have. The vertex conversion affects sphere and cylinder power, not these fitting parameters.
ISO 8980-1 & ANSI Z80.1 rounding standard
All converted contact lens values are rounded to the nearest 0.25 D — the internationally accepted increment for ophthalmic lenses per ISO 8980-1 and ANSI Z80.1. This is identical to the rounding applied in clinical optometry practice.
When conversion is essential
Who needs a vertex distance correction?
Anyone with a moderate to high prescription should always convert before ordering contact lenses online. Wearing uncorrected powers — especially past ±6.00 D — can accelerate eye strain and cause headaches that are easily preventable.
Vertex correction is also critical when switching between different frame styles that sit at different distances from the eye, or when a patient's vertex distance has been measured and differs from the standard 12 mm default.
Reference table
| Spectacle power | Conversion needed? | Typical change |
|---|---|---|
| 0.00 to ±3.75 D | Usually negligible | 0.00 – 0.25 D |
| ±4.00 to ±6.75 D | Clinically relevant | 0.25 – 0.50 D |
| ±7.00 to ±10.00 D | Always required | 0.50 – 1.00 D |
| Above ±10.00 D | Essential | 1.00 D or more |
Frequently Asked Questions
The most searched questions about converting eyeglass prescriptions to contact lenses.
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.
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 clinical formula Fc = Fs ÷ (1 − d × Fs) to calculate your contact lens power, rounded to the nearest 0.25 D per ISO 8980-1.
What formula does this calculator use?
The industry-standard vertex distance formula: Fc = Fs ÷ (1 − d × Fs), where Fs is the spectacle power and d is vertex distance in meters. Results are rounded to the nearest 0.25 D per ISO 8980-1 & ANSI Z80.1.
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. A −6.00 D spectacle lens placed 12 mm from the cornea delivers approximately −5.75 D of correction at the corneal plane. A contact lens sitting directly on the eye must be prescribed at that corrected value — not the glasses value.
Do daily, monthly, and toric contact lenses use the same converted power?
Yes. The vertex distance conversion applies equally to all lens types. Whether you wear daily disposables, monthly lenses, or toric contacts for astigmatism, the converted SPH and CYL values are identical. Lens type affects base curve and diameter — not the optical power calculation.
Does this tool handle astigmatism (CYL)?
Yes. The calculator converts both sphere and cylinder values. For spherical contact lenses, cylinder values of ±0.75 D or less are typically absorbed into the sphere. Higher cylinder values may require toric lenses.
Can I use this result to order contacts directly?
This tool provides a medically accurate estimate — not a legal prescription. Contact lenses also require base curve and diameter measurements that only a fitting exam can provide. Always confirm with your eye care professional.
Does vertex distance correction work for plus (hyperopia) prescriptions?
Yes, but in the opposite direction. For plus (hyperopic) prescriptions, moving the lens closer to the eye reduces effective power — so the contact lens value is typically slightly less plus than the glasses value. A +6.00 D spectacle prescription, for example, converts to approximately +5.75 D at the corneal plane.
My prescription is high (+/− 6.00 or above). Is the conversion still accurate?
The formula remains accurate at high powers, but the difference between spectacle and contact values becomes larger and more critical. For prescriptions above ±6.00 D, a professional fitting is strongly recommended to verify the result.
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. For this calculator, you need SPH and CYL; Axis does not affect the vertex distance conversion.
Back vertex power vs front vertex power?
Back vertex power (BVP) is the standard measurement used on most prescriptions — measured from the back surface of the spectacle lens. Front vertex power is used for high plus lenses above +5.00 D, where back vertex measurement becomes less accurate. This calculator supports both modes via the Advanced settings panel.
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.