Daily DAX : Day 386 WEEKNUM

Power BI DAX: WEEKNUM Function

Syntax

WEEKNUM(<date> [, <return_type>])

Parameters

Parameter Description
<date> A date in datetime format or a column containing dates.
<return_type> (optional) A number that determines the week numbering system:
1 or omitted: Week begins on Sunday (ISO week 1 contains Jan 4).
2: Week begins on Monday (ISO 8601 standard).
21: Week begins on Sunday (week 1 is the first week with 4+ days in the new year).

Return Value

An integer representing the week number of the year for the given date.

Use Cases

  • Weekly Sales Reporting: Group sales data by week for trend analysis.
  • Time Intelligence: Compare performance across weeks (e.g., Week 5 of 2024 vs. Week 5 of 2025).
  • Fiscal Calendar Alignment: Use return_type = 2 for ISO 8601 compliance in international reporting.
  • Dashboard Filters: Allow users to filter visuals by week number.

Examples

1. Basic Week Number (Sunday-based)

Week Number = WEEKNUM('Date'[Date])

Result: For Jan 1, 2025 (Wednesday) → Returns 1 (week starts on Sunday).

2. ISO Week Number (Monday-based)

ISO Week = WEEKNUM('Date'[Date], 2)

Result: For Jan 1, 2025 → Returns 1 (ISO week starts Monday; Jan 1–2 are in week 1 of 2025).

3. Calculated Column in a Date Table

Week Number ISO = WEEKNUM('Calendar'[Date], 2)

Used to create a proper fiscal or reporting week dimension.

Note: Always specify return_type = 2 for consistent international reporting (ISO 8601). Avoid default 1 unless your organization uses Sunday-start weeks.

Common Pitfalls

  • Using default return_type = 1 may cause misalignment with ISO standards.
  • Week 1 may include days from the previous year (especially with return_type = 21).

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