Glasses to Contacts Rx Calculator
Convert your glasses prescription to contact lens prescription using vertex distance adjustment for powers above ±4D.
About this calculator
A glasses lens sits some distance in front of the eye -- the Vertex Distance, typically around 12 mm -- while a contact lens sits directly on the cornea, and that difference in position changes the EFFECTIVE power the eye actually experiences even though the lens material itself hasn't changed. This calculator applies the standard vertex distance compensation formula, Adjusted Power equals Power divided by (1 minus vertex distance in meters times Power), to convert a glasses prescription into the equivalent contact lens power. The correction only meaningfully changes the result for stronger prescriptions -- by convention, at or beyond ±4 diopters -- because the formula's effect scales with the power itself; a weak prescription barely shifts at all when moved from spectacle plane to corneal plane, which is why this calculator leaves Sphere Power unchanged below that ±4 D threshold rather than applying a correction too small to matter in practice.
Cylinder Power uses the same method but gated on TOTAL power at the cylinder meridian (Sphere Power plus Cylinder Power combined), not on Cylinder Power's own magnitude alone -- a toric prescription like -10.00 SPH / -2.50 CYL has a small cylinder component on its own but a total meridian power of -12.50 D, well past the correction threshold, so gating on the cylinder value alone would silently skip a correction the prescription actually needs. Each Change output shows how far the adjusted value moved from the original glasses reading, rounded to the nearest 0.25 D step -- the standard increment contact lens powers are manufactured in. This calculator is an educational conversion tool only: your eye care provider performs the definitive glasses-to-contacts conversion (which may also factor in additional considerations specific to contact lens fitting), and any numbers here should be confirmed with them before ordering lenses.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Convert
Results
How to Use This Calculator
- Enter your Sphere Power (SPH) from your glasses prescription in diopters (e.g., −3.00 D for myopia).
- Enter your Cylinder Power (CYL) if your glasses correct astigmatism; enter 0 if none.
- Set the Vertex Distance — the distance from the back of your glasses lens to the cornea, typically 12 mm.
- The calculator adjusts Sphere Power when it exceeds ±4 D, and adjusts Cylinder Power when the total power at the cylinder meridian (sphere + cylinder) exceeds ±4 D.
- Review the Contact Lens Sphere and Contact Lens Cylinder outputs — these are the values to give your eye care provider.
How the result changes with Sphere Power (SPH)
| Sphere Power (SPH) | Contact Lens Sphere | Contact Lens Cylinder |
|---|---|---|
| -7.5 | -7 D | 0 D |
| -4.5 | -4.25 D | 0 D |
| -2.25 | -2.25 D | 0 D |
| -1.5 | -1.5 D | 0 D |
What each input means
- Sphere Power (SPH)
- Sphere power from your glasses prescription in diopters. Negative = myopia, positive = hyperopia.
- Cylinder Power (CYL)
- Cylinder (astigmatism) power from your glasses prescription in diopters. Enter 0 if none.
- Vertex Distance
- Distance from the back of the glasses lens to the cornea (typically 12 mm).
How this is calculated
Formula
Adjusted Power = Power / (1 - (vertex × Power))Worked example, using the default values
- Identify Input ParametersSphere Power (SPH) = -3, Cylinder Power (CYL) = 0, Vertex Distance = 12 = 3 input(s) provided
- Calculate Contact Lens SphereContact Lens Sphere-3 = -3
- Calculate Contact Lens CylinderContact Lens Cylinder0 = 0
- Calculate Sphere Change0 = 0
- Calculate Cylinder ChangeCylinder Change0 = 0
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why doesn't a weak prescription change at all when converted to contacts?
The vertex distance correction's size depends on the power itself -- mathematically, the adjustment scales with how strong the prescription is, so a weak prescription (below about ±4 diopters, by common convention) produces a correction too small to be clinically meaningful. This calculator leaves Sphere Power unadjusted below that threshold on its own magnitude, and leaves Cylinder Power unadjusted when the TOTAL power at the cylinder meridian (Sphere Power plus Cylinder Power) is below that threshold, rather than applying a fractional correction that wouldn't change the prescribed contact lens power in practice.
My Cylinder Power alone is under ±4 D -- why did it still get adjusted?
Because the relevant threshold for the cylinder correction is the TOTAL power at the cylinder meridian -- Sphere Power plus Cylinder Power combined -- not Cylinder Power's magnitude by itself. A prescription like -10.00 SPH / -2.50 CYL has a small cylinder value on its own, but the eye actually experiences -12.50 D at that meridian, well past the ±4 D threshold, so a correction is applied even though Cylinder Power alone looks small. This matches how Sphere Power's own correction is gated and keeps the two outputs internally consistent for the same toric prescription.
Does increasing Vertex Distance make the correction larger or smaller?
Larger. Vertex Distance measures how far the glasses lens sits in front of the cornea, and the compensation formula's adjustment grows with that distance -- a glasses lens sitting further from the eye (a larger Vertex Distance) needs a bigger power change to produce the same effective correction once moved onto the cornea as a contact lens.
Why is Contact Lens Sphere rounded to the nearest 0.25 D?
Contact lenses, like glasses lenses, are manufactured in standard power steps, and 0.25 diopters is the common increment for sphere and cylinder powers. Rounding the raw calculated adjustment to that step reflects the power a lab or manufacturer can actually produce, rather than reporting a precision the lens itself can't be made to.
Is a myopic (nearsighted) prescription adjusted the same way as a hyperopic one?
The same formula is applied to both, but the direction of the shift differs: for a myopic (negative-power) prescription, moving from glasses to contacts typically requires slightly LESS minus power once vertex distance is accounted for, while a hyperopic (positive-power) prescription typically requires slightly MORE plus power. This is a standard, well-established result of vertex distance optics, not something unique to this calculator.
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