MS Excel: ISOWEEKNUM function to return the ISO week number from a date
- Fakhriddinbek
- May 4
- 2 min read
In global business environments, especially across Europe and international organizations, week numbers often follow the ISO 8601 standard. Microsoft Excel offers the ISOWEEKNUM function to simplify working with ISO-standard week numbers.
This function is especially useful in project management, supply chain scheduling, financial planning, and any scenario where consistent week numbering is required across regions.
Syntax
=ISOWEEKNUM(date)
Parameter:
Argument | Description |
date | A valid Excel date or reference to a date |
Return:
A number from 1 to 53, representing the ISO week number of the year.
ISO week numbering: Weeks start on Monday, and week 1 is the week containing the first Thursday of the year.
Examples
Date | Formula | Result | Explanation |
2025-01-01 | =ISOWEEKNUM("2025-01-01") | 1 | Week 1, since Jan 1, 2025 falls on a Wednesday |
2025-01-05 | =ISOWEEKNUM("2025-01-05") | 1 | Same week as Jan 1, still week 1 |
2025-01-06 | =ISOWEEKNUM("2025-01-06") | 2 | New week begins (Monday), now Week 2 |
2024-12-30 | =ISOWEEKNUM("2024-12-30") | 1 | Falls into ISO week 1 of 2025 |
Use Cases
Scenario | Application |
Project timelines | Align deadlines and tasks with ISO weeks |
Supply chain logistics | Plan based on weekly cycles across regions |
Reporting and dashboards | Group KPIs or metrics by ISO week |
International planning | Coordinate with teams across time zones |
ISOWEEKNUM vs WEEKNUM
Feature | ISOWEEKNUM | WEEKNUM |
Standard | Follows ISO 8601 | Custom week numbering (system 1 or 2) |
Week starts on | Monday | Sunday (default) or Monday (system 2) |
First week of year | Week with the first Thursday | Week containing January 1st |
Consistency | International | May vary by region or Excel setup |
Use ISOWEEKNUM when consistency across countries and systems is needed.
Best Practices
Use ISOWEEKNUM for international reports or when collaborating with global teams.
Combine with YEAR to distinguish years:
excel
CopyEdit
=YEAR(A1) & "-W" & TEXT(ISOWEEKNUM(A1), "00")
➜ Returns something like "2025-W01"
For weekly summaries, you can group data in PivotTables using week numbers calculated with ISOWEEKNUM.
Summary Table
Feature | Description |
Function Name | ISOWEEKNUM |
Purpose | Returns the ISO week number of a date |
Output | Integer (1–53) |
Available In | Excel 2013 and later |
Week Start Day | Monday |
ISO Rule Used | Week 1 contains the first Thursday |
Final Thoughts
The ISOWEEKNUM function offers a reliable, standardized way to work with week numbers—especially critical in international operations and date-based reporting. It's a small but essential function that ensures data consistency, clarity, and professional formatting when working with time-sensitive projects in Excel.
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