Contact Lens Prescription Calculator
Estimate how your glasses prescription converts to contact lens power using the standard vertex distance formula — for educational understanding before your optician appointment.
Contact lens prescriptions require an in-person eye examination and contact lens fitting by a qualified optometrist or ophthalmologist. They must include base curve (BC) and diameter (DIA) measurements specific to your cornea, which cannot be calculated from a glasses prescription alone. Never order or wear contact lenses based solely on this calculator. Always obtain a valid contact lens prescription from a registered eye care professional.
Glasses Prescription Details
Enter the values from your spectacle prescription
Found on your prescription as “SPH” or “Sphere”. Negative = short-sighted (myopia), positive = long-sighted (hyperopia).
Distance from the back of your glasses lens to your cornea. Typically 12-14mm; ask your optician for your exact measurement.
Leave as 0 if you don’t have astigmatism correction on your prescription.
Only relevant if you entered a Cylinder value above (0-180°).
Estimated Contact Lens Power
Vertex distance converted value (Right Eye)
Enter your glasses prescription details above and click Calculate Estimated Power to see the vertex distance conversion.
Vertex Distance Conversion Examples
Examples of how glasses prescriptions typically convert to contact lens power at a standard 12mm vertex distance. These are illustrative only — your optician will confirm exact values.
| Glasses Power (SPH) | Approx. Contact Lens Power | Difference | Notes |
|---|---|---|---|
| -2.00 D | -2.00 D | None | Below ±4.00, conversion usually negligible |
| -4.00 D | -3.75 D | +0.25 D | Conversion threshold begins |
| -6.00 D | -5.50 D | +0.50 D | Common myopic correction |
| -8.00 D | -7.25 D | +0.75 D | High myopia |
| +2.00 D | +2.00 D | None | Below ±4.00, conversion usually negligible |
| +5.00 D | +5.25 D | -0.25 D | Hyperopia requires opposite adjustment |
| +7.00 D | +7.50 D | -0.50 D | High hyperopia |
Contact Lens Prescription FAQ
Everything you need to know about converting glasses prescriptions to contact lens power, and why a professional fitting is essential.
No, you cannot directly use a glasses (spectacle) prescription to order contact lenses. Contact lenses require a separate prescription that includes base curve (BC) and diameter (DIA) measurements, which are determined through a contact lens fitting with an optometrist. The lens power often also differs slightly from your glasses prescription due to vertex distance. Wearing incorrectly fitted contact lenses can cause discomfort, corneal abrasions, or infection.
Vertex distance is the gap between the back surface of your glasses lens and the front surface of your cornea, typically 12-14mm. Contact lenses sit directly on the eye with zero vertex distance, so for prescriptions above +/-4.00 dioptres, the effective power changes when moving from glasses to contacts. This is why a -6.00 glasses prescription might become approximately -5.50 in a contact lens.
Contact lens prescriptions differ from glasses prescriptions for several reasons: vertex distance correction changes the effective power, contact lenses require base curve (BC) and diameter (DIA) measurements specific to your cornea’s shape, and astigmatism correction (cylinder/axis) is often simplified or omitted in soft contact lenses below certain thresholds. A comprehensive contact lens fitting exam is required to determine the correct values.
In the UK, contact lens prescriptions are typically valid for 1 to 2 years, depending on your optician and the type of lenses you wear. Regular contact lens check-ups (usually annual) are required to assess eye health, corneal oxygenation, and ensure the prescription and fit remain appropriate, as your eyes can change over time.
No. Even if a vertex distance conversion suggests an approximate contact lens power, you should never wear contact lenses without a professional fitting. Opticians assess corneal curvature, tear film quality, and ocular health, all of which affect lens comfort and safety. Wearing the wrong base curve or diameter can cause serious eye damage even if the power is theoretically correct.
SPH (Sphere) indicates the lens power needed to correct short-sightedness (myopia, negative numbers) or long-sightedness (hyperopia, positive numbers), measured in dioptres. CYL (Cylinder) indicates the additional power needed to correct astigmatism. AXIS indicates the orientation (0-180 degrees) of the astigmatism correction. Not all prescriptions include CYL and AXIS — these only appear if you have astigmatism.