Enter any 2 of the cylinder power, sphere power, and spherical equivalent into the calculator to determine the unknown variable.

Spherical Equivalent Formula

The following formula is used to calculate the spherical equivalent.

SE = (C + (S/2))


  • SE is the spherical equivalent (diopters) C is the cylinder power (diopters) S is the sphere power (diopters)

To calculate the spherical equivalent, add the sphere power divided by 2 to the cylinder power. The result is the spherical equivalent in diopters.

What is a Spherical Equivalent?

A Spherical Equivalent is a simplified, single number representation of a person’s refractive error, or the power of an eyeglass prescription. It is calculated by adding half of the cylinder power to the sphere power in an eyeglass prescription. This measurement is often used in large population studies of refractive error, or when prescribing contact lenses, as it provides a general idea of the degree of nearsightedness or farsightedness. However, it does not account for astigmatism.

How to Calculate Spherical Equivalent?

The following steps outline how to calculate the Spherical Equivalent (SE).

  1. First, determine the cylinder power (C) in diopters.
  2. Next, determine the sphere power (S) in diopters.
  3. Next, use the formula SE = (C + (S/2)) to calculate the Spherical Equivalent.
  4. Finally, calculate the Spherical Equivalent (SE) in diopters.
  5. After inserting the values of C and S into the formula and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Cylinder power (C) = -2.50 diopters

Sphere power (S) = -3.75 diopters