Estimate spectacle prescription from contact lens power using vertex distance conversion for sphere, cylinder, and axis.
Enter your contact lens prescription values. This calculator estimates an equivalent glasses prescription at your selected vertex distance.
| Eye | Sphere (D) | Cylinder (D, optional) | Axis (1-180, if cyl used) |
|---|---|---|---|
| OD (Right) | |||
| OS (Left) |
Enter at least one eye and click “Convert to Glasses Rx”.
If you have a contact lens prescription and want to estimate what your glasses prescription might look like, you are asking a very common and very useful question. Contact lenses and glasses can correct the same refractive error, but they do not always use the exact same lens power. The reason is simple: they sit at different distances from the eye.
A contact lens rests directly on the tear film over the cornea. Glasses sit in front of the eye, usually around 10 to 14 millimeters away. This small distance, called vertex distance, changes how much effective power reaches the eye. For lower prescriptions this difference may be tiny. For moderate and especially high prescriptions, the difference can be significant enough that direct copying of contact powers into glasses can lead to blur, eyestrain, or undercorrection.
The optical effect of a lens depends not only on its power but also on where it is positioned relative to the eye. Moving a lens forward or backward changes the vergence of light at the corneal plane. Because glasses sit forward from the cornea, their labeled power may need adjustment to produce the same retinal focus as a contact lens.
As a rule of thumb:
These trends become more noticeable as power increases, especially beyond about ±4.00 D.
The calculator on this page uses a standard effective power relationship. If contact power is known at the corneal plane and you want the equivalent spectacle power at distance d meters from the cornea:
Fglasses = Fcontact / (1 + d · Fcontact)For example, with a 12 mm vertex distance (0.012 m):
After clinical rounding, that may be dispensed near -8.75 D or -9.00 D depending on the practitioner’s judgment, subjective refraction findings, and available lens options.
For toric prescriptions (sphere, cylinder, axis), conversion is performed by principal meridians:
This method is more accurate than trying to convert only spherical equivalent. Spherical equivalent can be helpful for rough planning, but it does not preserve full astigmatic detail for final spectacle correction.
This calculator provides an optical estimate. Real-world prescribing can differ due to binocular balance, accommodative status, visual demands, frame fit, lens design, over-refraction quality, and practitioner preference. In addition, manufacturers have discrete lens steps, and some powers may be prescribed differently to optimize comfort and performance.
If your prescription is complex, includes prism, multifocal parameters, keratoconus management, post-surgical optics, or significant anisometropia, rely on a licensed optometrist or ophthalmologist for final values.
| Contact Power (D) | Estimated Glasses Power (D) |
|---|
Can I use my contact prescription to buy glasses directly?
Usually no. Contacts and glasses prescriptions are related but not interchangeable, especially at higher powers.
What is a typical vertex distance?
About 12 mm is common, but actual frame fit can vary, and that affects precision.
Does this work for astigmatism?
Yes. The calculator converts by principal meridians and rebuilds sphere/cylinder/axis.
Is this accurate for very high prescriptions?
It gives a strong estimate, but high powers should always be professionally verified because small fitting differences can matter more.
Do I keep the same axis?
Generally yes in this optical conversion model, though final subjective refraction may alter axis slightly.