Glasses to contact lens converter

About this calculator

Glasses sit about 12 to 14 mm in front of the eye, so strong prescriptions need a slightly different power when the lens sits on the eye. This converter works out that power for each eye, including the cylinder for toric lenses.

What you need

• Your glasses prescription: SPH, and CYL and AXIS if they are filled in
• The vertex distance, if your prescription lists it (12 mm is filled in)

How it is calculated

Power at the eye = F ÷ (1 − d × F), where F is the glasses power in diopters and d is the vertex distance in metres. For astigmatism each principal meridian (sphere, and sphere + cylinder) is converted and rounded to the nearest 0.25 D; the contact cylinder is the difference between them and the axis stays the same.

What the result can and cannot tell you

Below about ±4.00 D the change is under 0.25 D, so contact lens power usually matches the glasses. In the US, contact lenses need their own prescription from a fitting, which adds the lens brand, base curve and diameter. Toric lenses come in limited cylinder and axis steps, so use this as a guide for the conversation with your fitter.

References

1. Vertex distance and effective lens power
2. Netra Eye Institute: Myopia

Related

• Visual acuity converter
• Glasses prescription reader
• Reading glasses strength estimator
• Myopia guide
• All eye calculators

For information only. These tools explain numbers and do not replace an eye examination or your eye doctor’s advice. Values you enter stay in your browser.