How to use it
- Enter the sphere and, if there is one, the cylinder and axis. Use the ± button to switch between plus and minus.
- Enter the vertex distance the prescription was measured at and the one it will be worn at, or tap Glasses → contacts (12 mm to 0 mm).
- Read the compensated power, rounded and exact, with each meridian worked out below.
Formula
Fc = F ÷ (1 − d × F), where F is the power in dioptres and d the distance the lens moves toward the eye, in metres (negative when it moves away). For a cylinder, work out the sphere meridian and the sphere-plus-cylinder meridian separately; the new cylinder is their difference.
Worked example
A −10.00 D spectacle prescription measured at 12 mm, fitted as a contact lens (0 mm): d = 0.012 m, so Fc = −10.00 ÷ (1 − 0.012 × −10.00) = −10.00 ÷ 1.12 = −8.93 D, or −9.00 D to the nearest 0.25. The same lens the other way round, a −8.00 D contact lens worn as glasses at 12 mm, needs −8.00 ÷ (1 − (−0.012) × −8.00) = −8.85 D.
Vertex conversion chart: glasses at 12 mm to contact lenses
Exact values from the formula above, and the nearest 0.25 D step, worked out by this calculator:
| Glasses (12 mm) | At the eye | Nearest 0.25 | Glasses (12 mm) | At the eye | Nearest 0.25 |
|---|---|---|---|---|---|
| −4.00 | −3.82 | −3.75 | +4.00 | +4.20 | +4.25 |
| −5.00 | −4.72 | −4.75 | +5.00 | +5.32 | +5.25 |
| −6.00 | −5.60 | −5.50 | +6.00 | +6.47 | +6.50 |
| −7.00 | −6.46 | −6.50 | +7.00 | +7.64 | +7.75 |
| −8.00 | −7.30 | −7.25 | +8.00 | +8.85 | +8.75 |
| −9.00 | −8.12 | −8.00 | +9.00 | +10.09 | +10.00 |
| −10.00 | −8.93 | −9.00 | +10.00 | +11.36 | +11.25 |
| −12.00 | −10.49 | −10.50 | +12.00 | +14.02 | +14.00 |
Minus powers get weaker at the eye and plus powers stronger, and the gap grows quickly with power: about 0.2 D at ±4.00 D but more than 1 D at ±10.00 D, which is why the correction matters most for high prescriptions.
References
- Brooks CW. Understanding Lens Surfacing. Butterworth-Heinemann, 1992, pp. 241–245 (effective power and vertex distance).
- Millodot M. Dictionary of Optometry and Vision Science, 8th ed. Elsevier, 2018: vertex distance normally 11–15 mm.
Formula checked on 7 October 2026. For eye-care professionals: confirm against the lens manufacturer's fitting guide.