There are two main ways to calculate differences or ratios between PivotTable values in Excel:

1. Show Values As:

This built-in feature allows you to display pre-defined calculations within your PivotTable itself. Here's how:

* Click on the value field you want to modify (e.g., Sales).
* From the dropdown arrow, select "Value Field Settings."
* In the settings window, under "Show Values As," choose the appropriate option:
* Difference from: This calculates the difference between the current value and the value of another item in the PivotTable (e.g., difference from previous month).
* % Difference from: This calculates the percentage difference between the current value and the value of another item.
* % of Parent Total: This displays the current value as a percentage of the parent category's total (e.g., Sales as a % of Total Sales by Region).
* % of Grand Total: This displays the current value as a percentage of the grand total for the entire PivotTable.

2. Calculated Fields:

This method allows you to create custom formulas within the PivotTable for more complex calculations. Here's how:

* Click on the "Analyze" tab in the PivotTable ribbon. (This tab might be hidden; if so, right-click anywhere on the PivotTable and check "Show PivotTable Analyze Tab.")
* In the "Fields, Items & Sets" group, click on "Calculated Field."
* In the "Name" field, enter a descriptive name for your calculation (e.g., "Sales Difference").
* In the "Formula" box, write your formula using existing PivotTable fields. Here are some examples:
* To calculate the difference between current month sales and previous month sales: `[This Month Sales] - [Previous Month Sales]`
* To calculate the percentage change between current month sales and previous month sales: `([This Month Sales] - [Previous Month Sales]) / [Previous Month Sales] * 100`
* Click "OK" to save your calculated field.
* The new field will appear in the PivotTable Fields list. You can drag it to the Values area to display the calculated results.

Remember:

* Whichever method you choose, ensure your PivotTable is set up with the necessary rows and columns to facilitate the calculations you want to perform.
* Calculated fields offer more flexibility but require formula knowledge.
* You can format the calculated values (differences or ratios) with appropriate number formats (e.g., percentage).