Glasses sit approximately 10–15mm away from your eyes (vertex distance), while contact lenses rest directly on the cornea. This distance changes the effective optical power reaching the eye. For prescriptions above ±4.00 diopters, this vertex distance creates a meaningful difference that must be compensated. The formula used is: F_CL = F_gl / (1 − d × F_gl), where d is the vertex distance in meters.

Vertex compensation becomes clinically significant at approximately ±4.00 diopters. Below this threshold, the difference is typically less than 0.25D and may not require adjustment. For example, a −3.00D glasses Rx converts to approximately −2.90D for contacts (rounds to −3.00D). However, a −8.00D glasses Rx converts to about −7.25D — a substantial 0.75D difference that definitely matters for clear vision.

Yes — the cylinder power is also compensated using vertex distance formulas applied to each principal meridian separately. The axis remains unchanged. If your cylinder power is low (typically ≤0.75D), some practitioners may recommend spherical equivalent contact lenses. For cylinder powers ≥1.00D, toric (astigmatism-correcting) contact lenses are generally recommended for optimal vision.

Eye prescriptions can be written in two formats: minus cylinder (used by most optometrists and all contact lens manufacturers) and plus cylinder (used by some ophthalmologists). They represent the same optical correction but express it differently. For example, −2.00 / −1.00 × 90° (minus cyl) is equivalent to −3.00 / +1.00 × 180° (plus cyl). Our tool accepts both — just toggle the format switch. The output always uses minus cylinder format, which is the standard for contact lenses.

The standard vertex distance is 12mm, which is the most common default used by optometrists and optical labs. However, actual vertex distance varies based on frame design and facial anatomy — ranging from about 10mm (close-fitting frames) to 15mm (extended-wear or safety glasses). Our tool lets you adjust this value. When in doubt, use 12mm or consult your optometrist, who can measure your actual vertex distance.

This tool provides an estimated contact lens prescription based on mathematical vertex compensation. However, a contact lens prescription also involves additional parameters like base curve, diameter, lens material, and brand-specific fitting characteristics. You should always obtain a valid contact lens prescription from a licensed eye care professional before ordering. Use this tool for educational purposes and to understand the relationship between your glasses and contact lens prescriptions.

The axis indicates the orientation of astigmatism correction and is an angular measurement, not a power measurement. Vertex distance compensation only affects optical power (diopters), not the angular orientation of the cylindrical correction. Therefore, the axis remains exactly the same when converting from glasses to contact lenses. The only time axis changes is during plus/minus cylinder format conversion, where it rotates by 90°.

The mathematical conversion using the vertex compensation formula is optically precise. Results are rounded to the nearest 0.25D, which matches the standard increment of commercially available contact lenses. However, real-world fitting involves additional factors: corneal curvature, tear lens dynamics, lens flexure, and individual comfort. An optometrist may fine-tune the final prescription during a contact lens fitting and evaluation.