Enter the total refractive astigmatism at the corneal plane and the refractive astigmatism at the anterior corneal to calculate the residual astigmatism.

## Residual Astigmatism Formula

The following formula is used to calculate a Residual Astigmatism.

RA = RACP – RAAC

- Where RA is the Residual Astigmatism
- RACP is the refractive Astigmatism at the corneal plane
- RAAC is the refractive Astigmatism at the anterior corneal

To calculate the Residual Astigmatism, subtract the refractive astigmatism at the anterior corneal from that at the corneal plane.

## What is Residual Astigmatism?

Definition:

A Residual Astigmatism is the astigmatism that remains after a procedure like lasik or the use of contacts.

## How to calculate Residual Astigmatism?

Example Problem:

The following example outlines how to calculate a Residual Astigmatism.

First, determine the refractive astigmatism at the corneal plane. In this example, this is measured to be 3.

Next, determine the refractive astigmatism at the anterior corneal. In this case, this is measure to be 2.5.

Finally, calculate the Residual Astigmatism using the formula above:

RA = RACP – RAAC

RA = 3-2.5

RA = .5