Contact Lens Conversion Calculator
Use this contact lens conversion calculator to estimate the contact lens power that matches your glasses prescription, adjusted for vertex distance.
Updated October 22, 2026
Medical Disclaimer: This calculator gives a general estimate for education only and cannot diagnose a condition. Share your result with your doctor or the clinician who ordered your test, and do not start, stop or change any medicine based on it. Read our full Medical Disclaimer.
How Contact Lens Power Is Calculated
Contact lens power is the lens strength needed at the surface of your eye. Glasses sit about 12 mm in front of your eye, so the same correction needs a slightly different power once the lens moves onto the eye². This calculator converts your glasses prescription to that eye-level power.
What Contact Lens Conversion Measures
The gap between the back of a glasses lens and the front of your eye is called the vertex distance. A lens acts a little stronger or weaker as it moves closer to the eye. Nearsighted eyes need a weaker minus power in a contact lens than in glasses. Farsighted eyes need a stronger plus power².
The Formula
Contact lens power = F / (1 - d x F)
F = glasses power in diopters (D), d = vertex distance in meters (12 mm = 0.012)
For astigmatism: convert the sphere, then convert sphere + cylinder, then cylinder at the eye = second result - first result
This is the standard effective power formula used in optometry, and astigmatism must be worked out one main direction (meridian) at a time¹.
How This Calculator Computes Your Result
Enter the sphere, cylinder and axis for each eye, and your vertex distance (12 mm by default, 10 to 15 mm). If your prescription lists one, use it.
If your cylinder is written as a plus number, we first rewrite it in minus cylinder form. This is the same prescription, written the way contact lenses for astigmatism are sold.
We convert the two main powers of each eye and rebuild the sphere and cylinder. The axis stays the same.
We also show the spherical equivalent (sphere plus half the cylinder), a single power that averages the two directions.
Results are rounded to the nearest 0.25 D, since contact lenses come in 0.25 D steps². You can switch to common soft lens steps instead, which use 0.50 D steps above 6.00 D for many brands⁷.
You also see the unrounded values and how much each power changed.
What Your Result Means
The higher your prescription, the bigger the change. These results are for a 12 mm vertex distance.
Glasses power | Power at the eye | Rounded contact lens power |
-2.00 | -1.95 | -2.00 |
-4.00 | -3.82 | -3.75 |
-6.00 | -5.60 | -5.50 |
-8.00 | -7.30 | -7.25 |
-10.00 | -8.93 | -9.00 |
+2.00 | +2.05 | +2.00 |
+4.00 | +4.20 | +4.25 |
+6.00 | +6.47 | +6.50 |
+8.00 | +8.85 | +8.75 |
Up to about 4.00 D, the change is under 0.25 D, so the contact lens power is usually the same as or one step from your glasses². Above that, the change grows quickly, and the exact vertex distance starts to matter too. A -15.00 prescription works out to about -13.04 at 10 mm but -12.24 at 15 mm.
For astigmatism, the cylinder changes too, which is why it cannot be converted on its own. Glasses of -7.00 -2.00 at a 13 mm vertex distance work out to about -6.42 -1.64 at the eye¹, which this calculator rounds to -6.50 -1.75. Soft lenses for astigmatism come in a few set cylinder powers, such as -0.75 to -2.75 in 0.50 D steps with axes every 10 degrees for one common brand⁷, so a fitted lens may differ.
How to Read Your Glasses Prescription
Your glasses prescription is usually a small table with a row for each eye³.
Term | Meaning |
OD | Right eye (oculus dexter) |
OS | Left eye (oculus sinister) |
SPH (sphere) | Main lens power. Minus means nearsighted, plus means farsighted |
CYL (cylinder) | Amount of astigmatism. Blank or DS means none |
AXIS | Direction of the astigmatism, from 1 to 180 degrees |
ADD | Extra power for reading, used in bifocals |
Prism | Correction that helps some people with double vision see one image |
ADD and prism are not part of this conversion. Multifocal contact lenses handle reading power in a different way, set at a fitting. For reading power on its own, see our Reading Glasses Calculator. Optometry guidance says a prescription over 5.00 D should note the vertex distance used¹, sometimes written as BVD (back vertex distance).
Why a Glasses Prescription Is Not a Contact Lens Prescription
Because contacts sit on the eye, a contact lens prescription is different from a glasses prescription and needs a separate fitting³. In the United States, the FTC Contact Lens Rule says a contact lens prescription must include the lens power, the material or manufacturer (or both), the base curve, and the diameter when needed⁴. A glasses prescription has a power but none of the lens details. Your eye care provider gives you a copy of your contact lens prescription after the fitting is complete, which can take more than one visit⁵. Contact lens sellers must have a copy of it or verify it with your prescriber⁵.
The FDA regulates all contact lenses as prescription medical devices and notes they are not "one size fits all"⁶. Contact lenses can lead to serious eye infections and corneal ulcers, which in rare cases cause blindness, so the FDA advises keeping lenses away from water and not sleeping in them unless your eye care provider says you can⁶. Use your result to understand your prescription, not to order lenses.
How Much Lens Power Changes at the Eye
The gap between your glasses power and the power at the eye, at a 12 mm vertex distance. Minus lenses get weaker at the eye and plus lenses get stronger, and the gap grows fast above about 4.00 D.
- Plus lenses: stronger at the eye by
- Minus lenses: weaker at the eye by
- One lens step (0.25 D)
Show data table
| Glasses power (D, minus or plus) | Plus lenses: stronger at the eye by (D) | Minus lenses: weaker at the eye by (D) | One lens step (0.25 D) (D) |
|---|---|---|---|
| 0.00 D | 0.00 | 0.00 | 0.25 |
| ±2.00 D | 0.05 | 0.05 | 0.25 |
| ±4.00 D | 0.20 | 0.18 | 0.25 |
| ±6.00 D | 0.47 | 0.40 | 0.25 |
| ±8.00 D | 0.85 | 0.70 | 0.25 |
| ±10.00 D | 1.36 | 1.07 | 0.25 |
A Note on Accuracy
This is an estimate of lens power only, and real vertex distance depends on your frame. Actual lens powers can vary between brands, and a fitter may start from the sphere or the spherical equivalent when astigmatism is low¹. The final power is confirmed on the eye at a contact lens fitting. Rigid gas permeable lenses form a layer of tears under the lens, so their power cannot be found this way. This is not a contact lens prescription.
Frequently Asked Questions
Is a contact lens prescription the same as a glasses prescription?
No. Because contacts sit on the eye, the power can differ, and a contact lens prescription also lists the lens brand or material, base curve and diameter from a fitting³ ⁴.
Can I buy contacts with my glasses prescription?
In the United States, sellers need a valid contact lens prescription, which your eye care provider gives you after a contact lens fitting⁵. A glasses prescription, or this calculator's result, does not meet that requirement⁴.
At what prescription does vertex distance matter?
Vertex distance matters most above about 4.00 D, because below that the change is under 0.25 D². At 12 mm, a -6.00 glasses prescription works out to about -5.60 at the eye (a -5.50 contact lens), and +6.00 to about +6.47 (+6.50).
How accurate is this contact lens conversion calculator?
The formula is the standard one used in optometry¹, so the math is reliable if your vertex distance is close to the real one. The final power can still change after an eye care provider checks the lens on your eye.
Sources
Spectacle and Ocular Refraction; Effective Power, Rabbetts, Optician
Vertex Distance Important for Many Power Calculations, Ocular Surgery News, 2005
How to Read an Eyeglasses Prescription, American Academy of Ophthalmology, 2023
Contact Lens Rule, 16 CFR Part 315, Federal Trade Commission
Buying Prescription Glasses or Contact Lenses: Your Rights, Federal Trade Commission, 2024
ACUVUE Technical Specification Guide, Johnson and Johnson Vision, 2025