Date & Time

ISOWEEKNUM Function in Excel

Returns the ISO week number of the year for a given date.

Syntax

  • =ISOWEEKNUM(date)

Arguments

  • date (required): Date

Examples

  • =ISOWEEKNUM("1/1/2024") - ISO week number - Result: 1

Working with dates in ISOWEEKNUM

  • Format cells as Date after formulas — serial numbers are normal underneath.
  • Use [TODAY](/functions/today/) and [NETWORKDAYS](/functions/networkdays/) for business calendars.
  • Guide: [Excel date functions](/guides/excel-date-functions-guide/).

Common errors

  • ISO weeks always start on Monday

Use cases

  • International reporting
  • ISO compliance
  • European standards

Frequently asked questions

  • Why does Jan 1 sometimes show week 52 or 53? ISO weeks belong to the year containing Thursday of that week. If Jan 1 is Fri/Sat/Sun, it belongs to the previous year's last week. This ensures consistent 7-day weeks across year boundaries.

Editorial review

  • Reviewed by Excel.Directory Editorial Team. Updated May 2026.

When to use ISOWEEKNUM

  • International reporting — common Date & Time scenario for ISOWEEKNUM.
  • ISO compliance — common Date & Time scenario for ISOWEEKNUM.
  • European standards — common Date & Time scenario for ISOWEEKNUM.

ISOWEEKNUM in the Date & Time category

  • Browse all Date & Time functions at /categories/date-time/ for related formulas.
  • ISOWEEKNUM syntax: =ISOWEEKNUM(date)
  • date (required): Date
  • Confirm ISOWEEKNUM arguments match the syntax shown above before filling down.
  • Lock table and range references with $ when copying formulas across rows or sheets.

Formula checklist before you copy down

  • Confirm ISOWEEKNUM arguments match the syntax shown above before filling down.
  • Lock table and range references with $ when copying formulas across rows or sheets.
  • If results look wrong, check for text stored as numbers and invisible spaces with TRIM.
  • Spot-check three known input rows manually against expected output.

Related Excel functions

  • WEEKNUM (/functions/weeknum/): Returns the week number of a specific date.
  • WEEKDAY (/functions/weekday/): Returns the day of the week for a date as a number.

Errors to watch for

  • ISO weeks always start on Monday — review causes on linked error pages in the directory.

Copy-paste audit workflow

  • Enter ISOWEEKNUM on three test rows with known expected output documented on a QA tab.
  • Fill down only after absolute references are locked on lookup tables and rate tables.
  • Compare against manual calculation or a calculator for financial and statistical functions.
  • Search this directory for comparison guides when choosing between similar functions in the same category.

ISOWEEKNUM worked examples to copy

  • =ISOWEEKNUM("1/1/2024") — ISO week number. Expected result: 1.

ISOWEEKNUM reference summary for crawlers and offline review

  • ISOWEEKNUM belongs to the Date & Time category in Excel. Returns the ISO week number of the year for a given date.
  • Full syntax: =ISOWEEKNUM(date). Open /functions/isoweeknum/ for parameters, FAQs, and related pages.
  • Common mistakes: ISO weeks always start on Monday
  • Pair this function with comparison guides when another Excel formula might fit the same task better.
  • Review fix-excel-formula-errors when unexpected errors appear after upgrading Excel or sharing across locales.