How do I get the week number in Excel?

Use =ISOWEEKNUM(TODAY()) for the current ISO week number. In older Excel versions, use =WEEKNUM(TODAY(),21) where return type 21 specifies ISO-8601.

What is the difference between ISO week numbers and US week numbers?

ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday. US weeks traditionally start on Sunday and week 1 is simply the week containing January 1. The two systems can differ by 1 week, especially in early January and late December.

What Week Number Is It? | Current Week Number 2026 ᐅ

Q: How is the ISO 8601 week number calculated?

Week 1 is the week containing the first Thursday of the year. Weeks start on Monday and end on Sunday. A year can have 52 or 53 weeks depending on what day of the week January 1 falls on.

Q: Can a week span two different calendar years?

Yes. The first few days of January can belong to week 52 or 53 of the previous year, and the last few days of December can belong to week 1 of the following year, depending on what day of the week they fall on.

Q: Which countries use ISO week numbering?

ISO week numbering is the standard in most of Europe, especially in Scandinavia, Germany, the Netherlands, and the UK. It is the default in most programming languages, databases, and enterprise software. The US and Canada traditionally use Sunday-based week numbering anchored to January 1.

Current Week Number

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Updates automatically every week

Year progress

Day of Year

Days Remaining

Quarter

Leap Year

All Week Numbers 2026

ISO 8601 week numbering · Weeks start on Monday

Week	From (Monday)	To (Sunday)

Programming Routines

Get the current ISO 8601 week number in your language

Excel Microsoft Excel / LibreOffice Calc

=ISOWEEKNUM(TODAY())

Or in older versions: =WEEKNUM(TODAY(),21) where 21 is ISO-8601. In Excel 2007 use WEEKNUM(TODAY(),2).

Sheets Google Sheets

=WEEKNUM(TODAY();21)

Type 21 is ISO-8601 — compatible with Excel and LibreOffice.

PHP PHP

$weekNumber = date("W");

Or date("W", epoch) for other dates. Use capital W, not w.

Python Python

datetime.date.today().isocalendar()[1]

Perl Perl

my $weekNumber = POSIX::strftime("%V", gmtime time);

Replace time with other epoch timestamps for other week numbers.

Java Java

Calendar now = Calendar.getInstance();
now.get(Calendar.WEEK_OF_YEAR);

Use WEEK_OF_YEAR from the Calendar class.

JS JavaScript

Date.prototype.getWeek = function () {
    var target  = new Date(this.valueOf());
    var dayNr   = (this.getDay() + 6) % 7;
    target.setDate(target.getDate() - dayNr + 3);
    var firstThursday = target.valueOf();
    target.setMonth(0, 1);
    if (target.getDay() != 4) {
        target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
    }
    return 1 + Math.ceil((firstThursday - target) / 604800000);
}

var d = new Date();
alert(d.getWeek());

Extend the Date prototype using the above code.

C# C#

CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(
    DateTime.Now,
    CalendarWeekRule.FirstFourDayWeek,
    DayOfWeek.Monday
);

MySQL MySQL

SELECT WEEKOFYEAR(NOW())

Or SELECT WEEK(NOW(), 3) using mode 3 for ISO-8601.

PostgreSQL PostgreSQL

SELECT EXTRACT(WEEK FROM CURRENT_DATE)

MS SQL MS SQL Server

SELECT DATEPART(wk, GETDATE())

Oracle Oracle

SELECT TO_CHAR(SYSDATE, 'IW') FROM DUAL

IW = ISO-8601 week (1–52 or 1–53). WW = week starting Jan 1 (mostly unused).

iSeries iSeries SQL

SELECT WEEK(NOW()) FROM sysibm.sysdummy1

Obj-C Objective-C (iPhone / Mac)

[NSString stringWithFormat:@"Week %d",
  [calendar ordinalityOfUnit:NSWeekCalendarUnit
                      inUnit:NSYearCalendarUnit
                     forDate:date]];

Swift Swift (iOS)

let gregorian = NSCalendar(calendarIdentifier:
    NSCalendarIdentifierGregorian)!
gregorian.firstWeekday = 2 // Monday
gregorian.minimumDaysInFirstWeek = 4
let components = gregorian.components(
    .WeekOfYearCalendarUnit | .YearForWeekOfYearCalendarUnit,
    fromDate: date)
let week = components.weekOfYear

R R

lubridate::week()

Ruby Ruby

week_number = Time.now.strftime("%V")

%U = week starting Sunday · %V = ISO-8601 · %W = week starting Monday.

Go Go

year, week := time.Now().ISOWeek()

Bash Linux / Bash

date +%V

Returns the ISO-8601 week number. See man date for more formats.

Lua Lua

Current_week = os.date("%V")

PowerShell Windows PowerShell

Get-Date -UFormat %V
# or
"{0:d2}" -f ($(Get-Culture).Calendar.GetWeekOfYear(
  $(Get-Date),
  [System.Globalization.CalendarWeekRule]::FirstFourDayWeek,
  [DayOfWeek]::Monday))

X++ X++ (Microsoft Dynamics AX)

int weeknum;
weeknum = weekOfYear(today());

C/AL C/AL (Microsoft Dynamics NAV)

MESSAGE(FORMAT(CALCDATE('CW', TODAY), 0, '<week>'));

Quick answer

The current ISO 8601 week number is displayed at the top of this page and updates automatically. Under ISO 8601, week 1 is the week containing the first Thursday of the year. Weeks run Monday to Sunday. Most years have 52 weeks; a year has 53 weeks when January 1 falls on a Thursday (or Wednesday in a leap year).

How ISO 8601 week numbers work

ISO 8601 is the international standard for representing dates and times, published by the International Organization for Standardization. It defines the week number using what is known as the first-Thursday rule: week 1 of the year is the calendar week that contains the first Thursday of January. Because a week runs Monday to Sunday, this is always the week that has at least 4 days in the new year.

The rule has one non-obvious implication: January 1 is not always in week 1. If January 1 falls on a Friday, Saturday, or Sunday, those days still belong to the last week of the previous year. Week 1 of the new year begins on the following Monday.

The first-Thursday rule explained

An easy way to remember the rule: find the first Thursday of the year. Whatever week that Thursday falls in is week 1. Working backward from that Thursday, the Monday four days earlier is the first day of week 1.

For example, if January 1 is a Wednesday, then that Wednesday is already in the week whose Thursday is January 2. That makes January 2 the first Thursday, so the week of December 29 to January 4 is week 1 - and December 29, 30, and 31 of the previous year are technically in week 1 of the new year.

ISO week numbering vs US week numbering

Two different week-numbering systems are in common use worldwide. The ISO system is the international standard; the US system is used in North America and some other countries. They differ in two ways: which day starts the week, and how week 1 is defined.

Property	ISO 8601	US / North American
Week starts on	Monday	Sunday
Week 1 defined as	Week containing first Thursday of the year	Week containing January 1
Minimum days in week 1	4	1
Where used	Europe, most enterprise software, most databases	United States, Canada, some legacy software
Excel function	=ISOWEEKNUM(date)	=WEEKNUM(date, 1) or =WEEKNUM(date, 2)
Can week numbers differ?	Yes - by up to 1 week, most often in early January and late December

The practical consequence: if you are sharing a week number with a US counterpart, confirm which system you are both using. "Week 1" can refer to different dates depending on the convention.

Years with 53 weeks

A calendar year has exactly 52 ISO weeks in most cases. A year has 53 ISO weeks when its January 1 falls on a Thursday (in non-leap years) or on a Wednesday or Thursday (in leap years). This happens roughly every 5 to 6 years. Week 53 always contains only a few days - typically 1 to 3 - bridging the gap between two calendar years.

Year	Jan 1 weekday	Leap year	ISO weeks
2020	Wednesday	Yes	53
2021	Friday	No	52
2022	Saturday	No	52
2023	Sunday	No	52
2024	Monday	Yes	52
2025	Wednesday	No	52
2026	Thursday	No	53
2027	Friday	No	52
2028	Saturday	Yes	52
2029	Monday	No	52
2030	Tuesday	No	52
2031	Wednesday	No	52
2032	Thursday	Yes	53
2037	Thursday	No	53
2043	Thursday	No	53

When week numbers are used

Business and project scheduling

Week numbers are the standard unit of planning in many industries, particularly manufacturing, logistics, and construction. Rather than specifying a date range, a production schedule might say "deliver in W34" - meaning the week beginning on the Monday of ISO week 34. This is more compact and less ambiguous than saying "the week of August 18" (which depends on which day the recipient considers the start of the week).

Most enterprise resource planning (ERP) systems, including SAP, Oracle, and Microsoft Dynamics, use ISO week numbers in planning views and reports. If you work with data exported from these systems, the week numbers will follow ISO 8601.

Scandinavia and Northern Europe

Week numbers are in everyday use in Sweden, Norway, Denmark, Finland, and the Netherlands in a way that is unusual elsewhere. People refer to vacations ("I'm off in week 28"), deadlines ("submit by week 12"), and meetings ("let's schedule in week 9") using week numbers in ordinary conversation. Public institutions, schools, and broadcasters publish schedules in week number format as a matter of course.

Agriculture and seasonal industries

Farming, fishing, and food processing industries use week numbers extensively for harvest and supply chain planning. A supplier contract might specify deliveries in "weeks 14-18" (the spring harvest window) rather than calendar dates, since the exact dates of a given week number are consistent year to year in terms of seasonal position.

Payroll and HR systems

Weekly payroll cycles are often tracked by ISO week number, particularly in European companies. A timesheet for "Week 22" is unambiguous regardless of the year or the employee's location. Many HR systems generate reports and pay runs keyed to ISO week numbers.

Common mistakes

Confusing ISO and US week numbers in software

The most common programming mistake with week numbers is using a function that returns US-style weeks (starting Sunday, with week 1 anchored to January 1) when ISO weeks are needed. In Excel, =WEEKNUM(date) returns US weeks by default. You must use =ISOWEEKNUM(date) or =WEEKNUM(date,21) for ISO weeks. In SQL, DATEPART(wk, date) in SQL Server returns US weeks; use DATEPART(isowk, date) for ISO.

Assuming week 1 starts on January 1

A date in early January is not always in week 1. If you filter data for "week 1" and expect to capture all records from January 1 onward, you will miss records from January 1-3 when those days fall on Friday through Sunday (because those days are in week 53 of the prior year). Always check the actual Monday-to-Sunday date range of week 1 before filtering.

Year boundary confusion

A date in late December can have an ISO year that is different from its calendar year. December 31, 2018 was in ISO week 1 of 2019 - its ISO year is 2019, not 2018. If you store week numbers alongside a calendar year column, the combination may be internally inconsistent for dates in late December or early January. Use the ISO year (the year that the week belongs to) alongside the week number, not the calendar year of the date.

FAQs

What is the current week number?

The current ISO 8601 week number is shown at the top of this page and updates automatically each week.

How is the ISO 8601 week number calculated?

Week 1 is the week containing the first Thursday of January. Weeks run Monday to Sunday. Count how many complete weeks have passed since that first Monday to get the week number for any date.

Why doesn't week 1 always start on January 1?

Because ISO 8601 defines week 1 by its Thursday, not its first day. If January 1 falls on a Friday, Saturday, or Sunday, those days belong to week 52 or 53 of the previous year. Week 1 of the new year starts on the following Monday.

What is the difference between ISO and US week numbers?

ISO weeks start on Monday; US weeks start on Sunday. ISO week 1 is the week with the first Thursday; US week 1 is the week containing January 1. They can differ by 1, especially in early January and late December.

How many weeks are in a year?

Most years have 52 ISO weeks. A year has 53 weeks when January 1 falls on a Thursday (non-leap year) or Wednesday or Thursday (leap year). 2026 is a 53-week year.

How do I get the ISO week number in Excel?

Use =ISOWEEKNUM(TODAY()). In older Excel versions without this function, use =WEEKNUM(TODAY(),21) - return type 21 specifies ISO-8601.

Can a week span two different calendar years?

Yes. The last few days of December can belong to week 1 of the next year, and the first few days of January can belong to week 52 or 53 of the previous year, depending on what day they fall on.

Which countries use ISO week numbering?

ISO week numbering is the standard across most of Europe, especially Scandinavia, Germany, the Netherlands, and the UK. It is the default in most programming languages, databases, and enterprise software. The US and Canada traditionally use Sunday-based week numbering anchored to January 1.

What Week Number Is It?

All Week Numbers 2026

Programming Routines

How ISO 8601 week numbers work

The first-Thursday rule explained

ISO week numbering vs US week numbering

Years with 53 weeks

When week numbers are used

Business and project scheduling

Scandinavia and Northern Europe

Agriculture and seasonal industries

Payroll and HR systems

Common mistakes

Confusing ISO and US week numbers in software

Assuming week 1 starts on January 1

Year boundary confusion

FAQs

What is the current week number?

How is the ISO 8601 week number calculated?

Why doesn't week 1 always start on January 1?

What is the difference between ISO and US week numbers?

How many weeks are in a year?

How do I get the ISO week number in Excel?

Can a week span two different calendar years?

Which countries use ISO week numbering?

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