Date & Time

WEEKNUM Function in Excel

Returns the week number of a specific date.

Syntax

  • =WEEKNUM(serial_number, [return_type])

Arguments

  • serial_number (required): Date
  • return_type (optional): 1=week begins Sunday, 2=Monday

Examples

  • =WEEKNUM("3/15/2024") - Week number - Result: 11

Working with dates in WEEKNUM

  • 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

  • Different results based on return_type

Use cases

  • Weekly reporting
  • Scheduling
  • Period analysis

Frequently asked questions

  • What's the difference between WEEKNUM and ISOWEEKNUM? WEEKNUM uses US convention (week 1 contains Jan 1). ISOWEEKNUM uses ISO 8601 (week 1 contains first Thursday of year, always starts Monday). For international business, use ISOWEEKNUM.
  • How do I group data by week? Use WEEKNUM in a pivot table or SUMIFS: =SUMIFS(Sales, WEEKNUM(Dates), WEEKNUM(A1)) sums sales for the same week. Combine with YEAR for multi-year data.

Editorial review

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

When to use WEEKNUM

  • Weekly reporting — common Date & Time scenario for WEEKNUM.
  • Scheduling — common Date & Time scenario for WEEKNUM.
  • Period analysis — common Date & Time scenario for WEEKNUM.

WEEKNUM in the Date & Time category

  • Browse all Date & Time functions at /categories/date-time/ for related formulas.
  • WEEKNUM syntax: =WEEKNUM(serial_number, [return_type])
  • serial_number (required): Date
  • return_type (optional): 1=week begins Sunday, 2=Monday
  • Confirm WEEKNUM 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 WEEKNUM 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

  • ISOWEEKNUM (/functions/isoweeknum/): Returns the ISO week number of the year for a given date.
  • WEEKDAY (/functions/weekday/): Returns the day of the week for a date as a number.
  • YEAR (/functions/year/): Returns the year of a date as a four-digit number.

Errors to watch for

  • Different results based on return_type — review causes on linked error pages in the directory.

Copy-paste audit workflow

  • Enter WEEKNUM 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.

WEEKNUM worked examples to copy

  • =WEEKNUM("3/15/2024") — Week number. Expected result: 11.

WEEKNUM reference summary for crawlers and offline review

  • WEEKNUM belongs to the Date & Time category in Excel. Returns the week number of a specific date.
  • Full syntax: =WEEKNUM(serial_number, [return_type]). Open /functions/weeknum/ for parameters, FAQs, and related pages.
  • Common mistakes: Different results based on return_type
  • 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.