Quick Answer

WEEKNUM Function

Returns the week-of-year number for a date.

Syntax

WEEKNUM

(serial_number, [return_type])

Parameters

Parameter	Description	Required
serial_number	The date serial to evaluate.	Required
return_type	Optional. Controls the week-start day: 1 (default) = week starts Sunday; 2 = week starts Monday; 11-17 select a specific start day; 21 = ISO 8601 (week starts Monday, week 1 is the one containing the first Thursday).	Optional

Basic Example

Week number of New Year 2024

=WEEKNUM(DATE(2024, 1, 1), 2)

Result1

2024-01-01 is a Monday. With return_type 2 (Monday start), the week containing Jan 1 is week 1.

Advanced Examples

Example 1: ISO 8601 week for a late-December date spill

A date that belongs to the previous ISO year

return_type 21 follows the ISO rule

=WEEKNUM(DATE(2021, 1, 1), 21)

Result: 53

2021-01-01 is a Friday that falls in the last ISO week of 2020, so it returns 53.

Example 2: US week starting Sunday

Comparing numbering systems

return_type 1 starts the week on Sunday

=WEEKNUM(DATE(2021, 1, 1), 1)

Result: 1

With return_type 1, the week containing Jan 1 is always week 1, so 2021-01-01 returns 1.

How WEEKNUM Works

WEEKNUM determines which week a date falls in based on the chosen week-start day. Week 1 always contains January 1. The ISO mode (21) defines week 1 as the week with the first Thursday (equivalently the week containing Jan 4), which can place early January dates in week 52 or 53 of the prior year.

1

Enter the date

Provide a date serial or a DATE(...) expression.

2

Choose return_type

Use 1/2 for US-style, or 21 for ISO 8601 weeks.

3

Read the week number

The function returns an integer 1-53.

Important Notes & Limitations

Week 1 always contains Jan 1, so early or late dates can return 52 or 53 under the ISO system.
For strict ISO weeks in Excel 2013 and later, the dedicated ISOWEEKNUM function is an alternative.
return_type must be 1, 2, 11-17, or 21; other values return #NUM!.

Common Errors & Fixes

#NUM!An invalid return_type value was supplied.

Fix: Use 1, 2, 11-17, or 21.

Week number seems off by oneMixing Sunday-start and Monday-start systems, or expecting ISO behavior.

Fix: Use return_type 21 for ISO 8601 weeks.

Download Practice File

Practice WEEKNUM with Real Data

Download a sample CSV file with pre-populated data and practice exercises for the WEEKNUM function. Works in both Excel and Google Sheets.

Works in Google SheetsCompatible with ExcelIncludes exercises

File format: CSV (comma-separated values) - opens in Excel, Google Sheets, and all spreadsheet apps

Frequently Asked Questions

How do I get ISO 8601 week numbers?

Use return_type 21, e.g. =WEEKNUM(date, 21).

Why is Jan 1 sometimes week 53?

Under ISO (21), if Jan 1 falls in the last week of the previous year, it returns 52 or 53.

What is the difference between return_type 1 and 2?

1 starts the week on Sunday, 2 starts it on Monday.