ISOWEEKNUM Function in Excel: ISO Week Number

ISOWEEKNUM Function in Excel: ISO Week Number featured illustration

📎 This article includes 1 downloadable practice file ↓

⏱ 2 min readUpdated 28 September 2026

DateLevel: IntermediateAvailable in: Excel 2013+

In this article
  1. Syntax
  2. Examples
  3. Example 1
  4. ISOWEEKNUM in practice
  5. Where you will use it
  6. Worked example
  7. Mistakes people make
  8. Related functions
  9. Continue learning

ISOWEEKNUM returns the ISO 8601 week (weeks start on Monday; week 1 holds the first Thursday). Use it when European teams or SAP reports say “week 1”, because WEEKNUM can disagree around New Year.

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Syntax

=ISOWEEKNUM(date)
Argument What it means
date A date.

Examples

Example 1

=ISOWEEKNUM(DATE(2016,1,1))

Returns 53 — 1 Jan 2016 belongs to the last ISO week of 2015.

ISOWEEKNUM in practice

Where you will use it

  • Week numbers for European clients
  • Weekly reporting that matches ISO calendars
  • Supply-chain planning

Worked example

DateFormulaResult
01-Jan-2026=ISOWEEKNUM(A2)1
04-Oct-2026=ISOWEEKNUM(A3)40
31-Dec-2026=ISOWEEKNUM(A4)53

Results calculated in Excel.

Mistakes people make

MistakeWhat to do instead
Comparing with WEEKNUMWEEKNUM starts weeks on 1 January; ISO weeks start on Monday
Week 53Some years have 53 ISO weeks

WEEKNUM · WEEKDAY

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All Date & Time guides →
TIMEVALUE Function in Excel: Text Time to Real TimeDateLevel: IntermediateAvailable in: Excel 2007+TIMEVALUE turns text like "2:45 PM" into a real time…DATEVALUE Function in Excel: Convert Text Dates Into Real DatesDATEVALUE turns a date written as text into a real date number. It follows…MINUTE Function in Excel: Minute From a TimeDateLevel: BeginnerAvailable in: Excel 2007+MINUTE returns 0–59 from a time.Syntax=MINUTE(serial_number)ArgumentWhat it.SECOND Function in Excel: Second From a TimeDateLevel: BeginnerAvailable in: Excel 2007+SECOND returns 0–59. Mostly used to strip seconds from machine.

📎 Practice files for this article

  • 📗
    Practice workbook: Dates & times22 tasks on realistic sales data — type your formula, the Check column turns green when it is right. Answers sheet included.
    ⬇ XLSX · 34 KB

Free to use for learning. Files with macros (.bas) are plain text — import them with Alt+F11 → File → Import File, and always test on a copy.

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