Spectacle to Contact Lens
Convert glasses Rx to starting soft lens power (vertex + optional spherical equivalent)
Convert a full spectacle prescription (sphere, cylinder and axis) to a contact lens power, with vertex correction applied to each meridian. Learn the maths.
Calculator
Enter spectacle Rx. Vertex compensation applies when powers exceed ±4 D. Use spherical equivalent only when fitting a spherical lens over low astigmatism.
OD
OS
Clinical notes
Moving a lens from the spectacle plane (~12 mm from cornea) to the corneal plane changes effective power. Minus Rx becomes less minus at the cornea; plus Rx becomes more plus. Vertex effects matter once powers exceed about ±4 D. Spherical equivalent (SE = Sphere + Cylinder/2) is a compromise for fitting spherical lenses over low regular astigmatism—use toric when full cylinder correction is needed.
Converting a spectacle prescription to a contact lens
Moving a prescription from spectacles to contact lenses is more than copying numbers across. Because a contact lens sits on the eye rather than ~12 mm in front of it, higher powers must be vertex-corrected, and for astigmatic prescriptions, each principal meridian converts by a slightly different amount. This calculator handles sphere and cylinder together so you get an accurate starting contact lens power.
Enter the spectacle sphere, cylinder and axis; the tool converts to the effective power at the corneal plane and presents the contact lens prescription. Treat the result as a starting point to confirm with a trial lens.
When does vertex correction actually change the lens?
Meaningfully at or above ±4.00 D in any meridian. Below that, the change is usually within one 0.25 D step. See the vertex distance guide.
Does it account for residual astigmatism?
The conversion handles the vertex effect on the refractive prescription. Lenticular (residual) astigmatism and lens rotation still need to be assessed clinically with over-refraction. See the toric fitting guide.
Read Vertex distance conversion explained for the underlying maths. Always verify with a trial-lens over-refraction. See our disclaimer.