Marco Russo and Alberto Ferrari explain one of the most important DAX functions:

CALCULATE, with its companion function CALCULATETABLE, is the only function in DAX that can change the filter context. Its use is very intuitive at first, and most DAX developers start using CALCULATE without knowing the most intricate details of its behavior. Then, sooner than later the use of CALCULATE becomes frightening because CALCULATE starts to misbehave. When this happens, it is nothing but a signal that you need to learn more theory and deepen your understanding of the behavior of CALCULATE.

In this article, we do not introduce the most complex behaviors of CALCULATE. Instead, we provide a beginner’s guide to CALCULATE, and we try to avoid making things simpler than they are. CALCULATE is definitely a complex function. Here we introduce its base behaviors, with a solid theoretical foundation.

Definitely a must-read if you work with Power BI and don’t have `CALCULATE()`

down pat.