Understanding Calculations in Tableau
This series introduces the basics of understanding calculations in Tableau. In this topic, you'll learn why and when to use calculations.
Why use calculations
Calculations allow you to create new data from data that already exists in your data source, as well as perform computations on your data. This allows you to perform complex analyses and add fields to your data source on your own and on the fly.
When to use calculations
The first hurdle to learning calculations in Tableau is to recognise when you actually need to use one. You can use calculations for many, many reasons. Some examples might include:
 To segment data
 To convert the data type of a field, such as converting a string to a date.
 To aggregate data
 To filter results
 To calculate ratios
Some common scenarios might include:

The data you need for your analysis is missing from your data source.
For example, if you have a Sales and Profit field in your data source, but you want to calculate cost, you can create a Cost field using a formula similar to the following.
[Sales]  [Profit]

You want to transform values in your visualisation.
For example, you might want to calculate the difference in profit from one year to the other. You can use a quick table calculation to show the difference in profit in the visualisation.

You want to quickly categorise data.
For example, you might want to quickly colour the data in your visualisation as profitable or nonprofitable. You can create a calculated field using a calculation similar to the following and then add it to Colour on the Marks card.
IF SUM([Profit]) > 0
THEN "Profitable"
ELSE "Nonprofitable"
END