Spherical Equivalent Calculator
SE = Sphere + Cyl/2
Calculate the spherical equivalent of a sphero-cylindrical prescription. Used for contact lens fitting, refractive surgery, and comparing prescriptions.
What is Spherical Equivalent?
Spherical Equivalent (SE) converts a sphero-cylindrical prescription into a single spherical power that represents the average refractive error. It's the power at the circle of least confusion (midpoint between the two focal lines).
SE = Sphere + (Cylinder / 2)
Works with both plus and minus cylinder formats
Common Uses:
- •Soft contact lens fitting - When cylinder is <0.75D, spherical CLs may work
- •Refractive surgery - Comparing pre/post-op refractive status
- •IOL calculations - Selecting intraocular lens power
- •Research - Comparing refractive error across studies
- •Quick estimates - Approximating vision without full correction
Important Note
Spherical equivalent is an approximation. Patients with significant astigmatism (cylinder >1.00D) will likely notice blur with SE-only correction. The full sphero-cylindrical prescription provides best visual acuity.
Example Calculations
Example 1: Minus Cylinder
-2.00 -1.50 x 180
SE = -2.00 + (-1.50 / 2) = -2.00 + (-0.75) = -2.75 D
Example 2: Plus Cylinder
-3.50 +2.00 x 090
SE = -3.50 + (+2.00 / 2) = -3.50 + (+1.00) = -2.50 D
Example 3: Sphere Only
+1.25 sphere
SE = +1.25 + (0 / 2) = +1.25 D (no change)