Vision Prescription Converter

An eyeglass prescription contains several values that together define the corrective lenses you need. Understanding what each number means helps you compare prescriptions, understand your lens power, and make informed choices when ordering glasses or contacts.

Key formula:

Spherical Equivalent (SE) = Sphere + (Cylinder ÷ 2)

This single number summarizes your overall refractive correction and is used for soft contact lens prescriptions.

Vertex Distance Correction (for strong prescriptions): True Power = P ÷ (1 − d × P) where d = vertex distance change in meters (typically 0.012 m = 12 mm)

What each prescription term means:

• Sphere (SPH) — the primary correction strength in diopters. Negative = nearsighted (myopia); positive = farsighted (hyperopia). Range: typically −20.00 to +20.00 D.
• Cylinder (CYL) — astigmatism correction strength. Always negative in minus-cylinder notation (US standard); always positive in plus-cylinder notation (European standard).
• Axis — the angle of the cylinder correction, in degrees from 1 to 180. Only meaningful when CYL is present.
• Add — additional magnification for reading (presbyopia). Typically +0.75 to +3.50 D. Present on bifocal or progressive prescriptions.
• Prism — corrects eye alignment issues (measured in prism diopters). Not always present.

Worked example: Right eye: −3.25 SPH, −1.00 CYL, Axis 175, Add +2.00 SE = −3.25 + (−1.00 ÷ 2) = −3.25 − 0.50 = −3.75 D spherical equivalent

Reading power = −3.25 + 2.00 = −1.25 D (distance prescription + add power)

Prescription strength reference:

• Mild: ±0.50 to ±2.00 D | Moderate: ±2.25 to ±5.00 D | Strong: ±5.25 to ±9.00 D | Very strong: > ±9.00 D