A client's spectacle prescription is written at -6.50D and they're asking what that translates to for daily contact lenses — before answering, you want the vertex-corrected figure, not just the raw number.
Contact lens power = Spectacle power ÷ (1 − Vertex distance(m) × Spectacle power)
As a lens moves closer to the eye (from spectacle plane to corneal plane), its effective power changes. This formula corrects for that shift — the effect is negligible for low powers but significant beyond roughly ±4.00D.
1 What this calculator does
Converts a spectacle lens prescription power to the equivalent contact lens power, correcting for the vertex distance (the gap between the back of a spectacle lens and the front of the eye, typically ~12mm) that doesn't apply when a lens sits directly on the cornea.
2 Formula & professional reasoning
Contact lens power = Spectacle power / (1 - Vertex distance(m) x Spectacle power)
A lens's effective power at the eye changes depending on how far it sits from the eye's optical system. Spectacle lenses sit roughly 12mm in front of the cornea (the vertex distance), while contact lenses sit directly on it. For low-power prescriptions this shift makes little practical difference, but for moderate-to-high myopia or hyperopia, failing to correct for vertex distance can produce a meaningfully wrong contact lens power — under-correcting myopia or over-correcting hyperopia (or vice versa depending on sign), affecting visual comfort and clarity.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
CL power = -2.00/(1-0.012x-2.00) = -2.00/1.024 = -1.95DCL power = -6.50/(1-0.012x-6.50) = -6.50/1.078 = -6.03DCL power = 8.00/(1-0.014x8.00) = 8.00/0.888 = +9.01D4 Sanity check
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| Skipping vertex correction for high prescriptions | Assuming contact lens power always equals spectacle power regardless of prescription strength | Meaningful under- or over-correction for moderate-to-high prescriptions, affecting visual comfort | Apply vertex distance correction for any prescription at or beyond ±4.00D |
| Using an assumed vertex distance instead of a measured one | Defaulting to 12mm for every client without measuring | Introduces avoidable error, particularly for high-power prescriptions where the correction is most impactful | Measure actual vertex distance with a distometer where precision matters, especially for prescriptions beyond ±5.00D |
| Applying the formula to cylindrical (astigmatism) power without adjustment | Using this simple spherical-equivalent formula directly on cylinder power without considering meridian-specific effects | Oversimplifies the correction for significant astigmatism, which needs meridian-by-meridian vertex correction | For significant cylindrical correction, use a full spherocylindrical vertex conversion (or reference table) rather than this simplified spherical formula |
| Forgetting to round to an available manufactured power | Prescribing the exact calculated decimal power to the client | Most contact lens ranges don't manufacture every decimal power — the exact calculated value usually isn't orderable | Round the calculated result to the nearest power step actually available from the lens manufacturer/range being used |
6 Reference & regulatory links
7 Professional workflow
Common tools used alongside this one: