DAX, short for Data Analysis Expressions, is the formula language used by Power BI for calculations. It is a collection of functions, operators and values that work out new figures from the data already loaded into a report. DAX is used mainly to create measures and calculated columns.
What DAX is
DAX is a formula language. Microsoft's DAX basics guide describes it as "a collection of functions, operators, and constants that can be used in a formula, or expression, to calculate and return one or more values". In plain terms, DAX is how you tell Power BI to work something out.
It's not unique to Power BI. The same language is used in Power Pivot in Excel and in Analysis Services, so DAX skills carry across Microsoft's data tools. Within Power BI, DAX is written in Power BI Desktop, the free Windows app where reports are built.
What DAX is used for
You can import data and build useful charts in Power BI without writing a single DAX formula. DAX earns its place the moment a question needs real calculation. Microsoft's guide gives the typical examples: growth percentages across product categories, or year-on-year growth compared with market trends.
The job DAX does is to turn raw figures into business answers. A sales table holds transactions; DAX turns them into total revenue, an average order value, a running total or a comparison with last quarter. It creates new information from data that's already in the report.
Where DAX is written: measures and calculated columns
DAX formulas live in two main places, and the difference between them matters. Microsoft's DAX overview sets them out:
- Measures: calculations whose result changes with context. A Total Sales measure shows the right total for whatever a report is filtered to, whether that is one product, one region or everything. Measures are the workhorse of DAX.
- Calculated columns: a new column added to a table, with a value worked out for every row and stored in the model. A column combining year and quarter into "2026 Q1" is a calculated column.
Both are typed into the formula bar in Power BI Desktop, and every DAX formula begins with an equals sign. Choosing the right one is an early hurdle for most learners: our measures vs calculated columns entry sets out how they differ and when to use each. DAX also drives other parts of a model, such as calculated tables and row-level security rules.
How DAX compares to Excel formulas
DAX looks reassuringly familiar if you know Excel. Microsoft notes that many DAX functions "will appear very similar" to Excel formulas, and functions such as SUM behave much as you'd expect. Anyone comfortable with Excel formulas has a real head start.
The key difference is what a formula points at. An Excel formula usually works on a cell or a range of cells. A DAX function works on a whole column or table inside a data model, and a calculation can change as a report is filtered. DAX runs fastest when that model is built as a star schema, the table layout Microsoft recommends. That shift, from cells to columns, is the main thing to learn when moving from Excel to DAX.
DAX is where Power BI gets genuinely powerful Simple measures are quick to pick up; the concept of context, which decides how a calculation behaves, is where most people get stuck. Our two-day, hands-on Power BI Masterclass teaches DAX the practical way, building real measures rather than memorising syntax. If you are still comparing courses, our guide to choosing Power BI training in the UK covers what good DAX teaching should include.
