# What is the washer method formula?

##### 1 Answer

The general "formula" is integrate (area of large disk - area of small disk) times thickness.

#### Explanation:

Any attempt to give a more specific "formula" looks very complicated to me.

Here are a couple of pointers that may be helpful:

**For the Washer Method**

The area of a disk is the area of a circle. The volume is the area

So, for some

Where

The functions used for the radii, must have the same independent variable as the differential.

If revolving around a horizontal line

In this case:

The larger disk will be determined by the function

If revolving around a vertical line

In this case:

The larger disk will be determined by the function

If the graphs of the functions cross each other of the line about which we are rotating, that must be taken into account.

If two region must be broken into two or more pieces to, then the volume must be calculated by using two or more integral.