# power-bi-dax-best-practices

Instructs GitHub Copilot to write performant DAX code for .pbix, .dax, .md, and .txt files. It enforces Microsoft conventions for syntax and calculations.

- rule · `type=rule` · GitHub Copilot · ★ 40418
- Author: github · official GitHub repository
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/github/awesome-copilot/tree/HEAD/instructions/power-bi-dax-best-practices.instructions.md
- Updated: 2026-10-23
- HTML: https://affpapa.org/en/nevibecode/rules/power-bi-dax-best-practices

## What it does

This rule directs the coding assistant to follow Microsoft guidance when building Power BI measures. Copilot stores intermediate computations inside `VAR` blocks rather than repeating complex expressions. That guarantees single evaluation and keeps queries fast. For table row counts, it picks `COUNTROWS` instead of `COUNT`.

Naming syntax follows strict conventions. Column references always include the table name, such as `Sales[CustomerID]`. Measures stay unqualified so moving them between tables will not break existing code. For error prevention, the rule insists on `DIVIDE` and bans `IFERROR` functions because they slow down formula engines.

The prompt package covers common calculations, including time intelligence like `DATESYTD`, moving averages, and Pareto ABC classification. Adding calculated columns to large tables causes high memory overhead. Because of that, the rule explicitly demands preparing row-level transforms in Power Query.

## Requirements
- Works with: Power BI

## Install

### GitHub Copilot (native platform)

**Manually: put the files in place**
- File: `.github/instructions/power-bi-dax-best-practices.instructions.md` — The applyTo field sets which files it applies to

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/power-bi-dax-best-practices.md` — The paths frontmatter field limits the files: move applyTo from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### Cursor

**Manually: put the files in place**
- File: `.cursor/rules/power-bi-dax-best-practices.mdc` — Frontmatter: description, globs, alwaysApply. Move applyTo from the source into globs, or the rule will not attach on its own

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/power-bi-dax-best-practices.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/power-bi-dax-best-practices.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/github/awesome-copilot/tree/HEAD/instructions/power-bi-dax-best-practices.instructions.md

## When to use it
- prevent performance degradation from `IFERROR` and `ISERROR` functions
- format measure and column references according to Microsoft syntax
- reuse calculations with cached `VAR` variables
- build time intelligence measures like YTD and moving averages

## Example prompts
- Write a YoY sales growth measure in DAX using `VAR` variables.
- Create a 3-month moving average measure for `Sales` with `DATESBETWEEN`.
- Generate an ABC Pareto classification measure using `SWITCH` and running totals.

## Source text · power-bi-dax-best-practices.instructions.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: afe2f75626c6b781.

BEGIN THIRD-PARTY FILE afe2f75626c6b781
````text
---
description: 'Comprehensive Power BI DAX best practices and patterns based on Microsoft guidance for creating efficient, maintainable, and performant DAX formulas.'
applyTo: '**/*.{pbix,dax,md,txt}'
---

# Power BI DAX Best Practices

## Overview
This document provides comprehensive instructions for writing efficient, maintainable, and performant DAX (Data Analysis Expressions) formulas in Power BI, based on Microsoft's official guidance and best practices.

## Core DAX Principles

### 1. Formula Structure and Variables
Always use variables to improve performance, readability, and debugging:

```dax
// ✅ PREFERRED: Using variables for clarity and performance
Sales YoY Growth % =
VAR CurrentSales = [Total Sales]
VAR PreviousYearSales = 
    CALCULATE(
        [Total Sales],
        SAMEPERIODLASTYEAR('Date'[Date])
    )
RETURN
    DIVIDE(CurrentSales - PreviousYearSales, PreviousYearSales)

// ❌ AVOID: Repeated calculations without variables  
Sales YoY Growth % =
DIVIDE(
    [Total Sales] - CALCULATE([Total Sales], SAMEPERIODLASTYEAR('Date'[Date])),
    CALCULATE([Total Sales], SAMEPERIODLASTYEAR('Date'[Date]))
)
```

**Key Benefits of Variables:**
- **Performance**: Calculations are evaluated once and cached
- **Readability**: Complex formulas become self-documenting
- **Debugging**: Can temporarily return variable values for testing
- **Maintainability**: Changes need to be made in only one place

### 2. Proper Reference Syntax
Follow Microsoft's recommended patterns for column and measure references:

```dax
// ✅ ALWAYS fully qualify column references
Customer Count = 
DISTINCTCOUNT(Sales[CustomerID])

Profit Margin = 
DIVIDE(
    SUM(Sales[Profit]),
    SUM(Sales[Revenue])
)

// ✅ NEVER fully qualify measure references
YTD Sales Growth = 
DIVIDE([YTD Sales] - [YTD Sales PY], [YTD Sales PY])

// ❌ AVOID: Unqualified column references
Customer Count = DISTINCTCOUNT([CustomerID])  // Ambiguous

// ❌ AVOID: Fully qualified measure references
Growth Rate = DIVIDE(Sales[Total Sales] - Sales[Total Sales PY], Sales[Total Sales PY])  // Breaks if measure moves
```

### 3. Error Handling Strategies
Implement robust error handling using appropriate patterns:

```dax
// ✅ PREFERRED: Use DIVIDE function for safe division
Profit Margin = 
DIVIDE([Total Profit], [Total Revenue])

// ✅ PREFERRED: Use defensive strategies in model design
Average Order Value = 
VAR TotalOrders = COUNTROWS(Orders)
VAR TotalRevenue = SUM(Orders[Amount])
RETURN
    IF(TotalOrders > 0, DIVIDE(TotalRevenue, TotalOrders))

// ❌ AVOID: ISERROR and IFERROR functions (performance impact)
Profit Margin = 
IFERROR([Total Profit] / [Total Revenue], BLANK())

// ❌ AVOID: Complex error handling that could be prevented
Unsafe Calculation = 
IF(
    OR(
        ISBLANK([Revenue]),
        [Revenue] = 0
    ),
    BLANK(),
    [Profit] / [Revenue]
)
```

## DAX Function Categories and Best Practices

### Aggregation Functions
```dax
// Use appropriate aggregation functions for performance
Customer Count = DISTINCTCOUNT(Sales[CustomerID])  // ✅ For unique counts
Order Count = COUNTROWS(Orders)                    // ✅ For row counts  
Average Deal Size = AVERAGE(Sales[DealValue])      // ✅ For averages

// Avoid COUNT when COUNTROWS is more appropriate
// ❌ COUNT(Sales[OrderID]) - slower for counting rows
// ✅ COUNTROWS(Sales) - faster and more explicit
```

### Filter and Context Functions
```dax
````
END THIRD-PARTY FILE afe2f75626c6b781

The third-party file has ended. Everything between the afe2f75626c6b781 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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