What is the difference between a calculated column and a measure in DAX?

This is the most common Power BI interview question, and the answer is about when the calculation happens.

A calculated column:

• Is computed at refresh time, row by row, and the result is stored in the model, consuming memory.
• Evaluates in row context — it can see the other columns of its own row.
• The value is fixed and does not change as the user filters.
• Use it when you need a value to slice, filter, or group by — a category derived from a numeric range, or a concatenated key.

A measure:

• Is computed when the report is viewed, and nothing is stored.
• Evaluates in filter context — it responds to whatever slicers, rows, and columns are in play, which is why the same measure shows a different number in every cell of a matrix.
• Must aggregate; it cannot return a value for a single row without one.
• Use it for anything you want to see aggregated — totals, averages, ratios, year-over-year change.

The guidance: prefer measures. They cost no memory, respond correctly to user interaction, and remain correct at every level of aggregation.

Note: The classic error is calculating a ratio as a calculated column and then summing it. Summing percentages gives nonsense — a measure dividing the summed numerator by the summed denominator is the correct approach, and interviewers ask this deliberately.