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).
- ISO 8601 week numbering is the default in most enterprise software - SAP, Oracle, Microsoft Dynamics, PostgreSQL, Python, and Go all use it out of the box, not just European calendars.
- Week 1 is defined by its Thursday, not January 1. Days in early January can legally belong to week 52 or 53 of the prior year.
- 2026 is a 53-week year because January 1, 2026 falls on a Thursday. The next 53-week year is 2032.
- The US system starts weeks on Sunday and anchors Week 1 to January 1 - the two systems can show a different week number for the same date, most often in early January and late December.
- Late-December dates carry an ISO year one ahead of the calendar year. Mixing both in a database produces invisibly inconsistent data in annual reports.
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 numbers explained to a beginner
Think of ISO week numbers like numbered matchdays in a soccer league. The league doesn't schedule games by calendar date - it says "Matchday 14." Every club knows exactly which Monday-to-Sunday window that covers, without anyone needing to specify "the week of April 6".
ISO week numbering works the same way: "deliver by Week 34" is precise and unambiguous regardless of the calendar month.
The part that surprises most people is Week 1. In 2021, January 1 fell on a Friday. Those first three days of January (Friday, Saturday, Sunday) were still part of Week 53 of 2020, because a full ISO week must run Monday to Sunday. Week 1 of 2021 didn't begin until Monday, January 4.
A deadline set for "Week 1 of 2021" meant you had until Sunday, January 10 - not January 3. Use the date calculator to check the exact date range for any week number.
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 primarily used in the United States and parts of Canada and Mexico.
Referencing week number is not so common in day-to-day life in North America as it is in Europe, where people use week numbers on a daily basis.
The two ways of counting week numbers 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.
Use the date calculator to verify the exact Monday-to-Sunday range for any week number before scheduling.
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.
Within a 400-year Gregorian cycle, exactly 71 years have 53 ISO weeks and the remaining 329 years have 52. That ratio - 71 out of 400 - repeats precisely every four centuries, so the pattern of long years is entirely predictable. 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 and where 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. When comparing exported reports between two periods, the code diff checker can help spot discrepancies in raw data files.
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.
As a Dane, I've noticed that the week-number convention in Scandinavia takes real adjustment for people coming from outside the region. The first time I said "see you in week 32" to a colleague with no further clarification, he got very confused and had to count backwards on his fingers.
After a few months it usually becomes second nature for people - week 32 feels as natural a time reference as "late July." The key shift is realizing the week number is the primary reference, not a secondary label attached to a date range.
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.
If you need to calculate the number of days or working days within a payroll period, the date calculator covers that directly.
Common mistakes when checking week numbers
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 and Google Sheets, =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.
So I definitely recommend using the ISO year (the year that the week belongs to) alongside the week number, not the calendar year of the date.
In my experience, this ISO year vs. calendar year mismatch is one of the most common silent data quality issues in business reporting.
I've seen annual summaries where the last three days of December were misattributed to the wrong year, making year-end totals look slightly (sometimes completely) off with no obvious cause.
The fix is straightforward - derive the ISO year from the date itself using the database's native ISO week year function rather than the calendar year column - but the bug is invisible until you happen to check a 53-week year and notice the final week is missing from one annual bucket.
How to find the date for a week number
The tool above shows the date range for every week of the current year. But if you need to calculate the Monday start date for any week number manually - for a past or future year - the formula below always works.
The formula: three steps
- Find January 4 of the target year. January 4 is always in ISO Week 1 by definition - it is the earliest date that can never fall in the previous year's last week.
- Find the Monday of that week. Subtract the ISO weekday number of January 4 minus 1. (Monday = 0 days back, Tuesday = 1 day back, ..., Sunday = 6 days back.)
- Add (week - 1) × 7 days. The result is the Monday that starts your target week. Add 6 more days for the Sunday end date.
Worked example: Week 34, 2026
January 4, 2026 falls on a Sunday (ISO weekday 7). Subtract 6 days: the Monday of Week 1 is December 29, 2025. Add (34 - 1) × 7 = 231 days to December 29, 2025.
The result: Week 34, 2026 runs Monday August 17 to Sunday August 23. The week number table at the top of this page confirms this directly.
Why this matters for date filtering in code
If your database query filters by week number alone, you may pull records from the wrong year at year boundaries. Always derive the actual Monday date using this formula and filter on a date range instead of a bare week number. The date calculator handles the arithmetic if you prefer not to compute it by hand.
FAQs about week of the year
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.
Week number in programming routines
Get the current ISO 8601 week number in your code language
=ISOWEEKNUM(TODAY()) =WEEKNUM(TODAY(),21) where 21 is ISO-8601. In Excel 2007 use WEEKNUM(TODAY(),2). See our Excel file diff checker →=WEEKNUM(TODAY();21) $weekNumber = date("W"); date("W", epoch) for other dates. Use capital W, not w.my $weekNumber = POSIX::strftime("%V", gmtime time); time with other epoch timestamps for other week numbers.Calendar now = Calendar.getInstance();
now.get(Calendar.WEEK_OF_YEAR); WEEK_OF_YEAR from the Calendar class.Date.prototype.getWeek = function () {
var target = new Date(this.valueOf());
var dayNr = (this.getDay() + 6) % 7;
target.setDate(target.getDate() - dayNr + 3);
var firstThursday = target.valueOf();
target.setMonth(0, 1);
if (target.getDay() != 4) {
target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
}
return 1 + Math.ceil((firstThursday - target) / 604800000);
}
var d = new Date();
alert(d.getWeek()); CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(
DateTime.Now,
CalendarWeekRule.FirstFourDayWeek,
DayOfWeek.Monday
); SELECT WEEKOFYEAR(NOW()) SELECT WEEK(NOW(), 3) using mode 3 for ISO-8601.SELECT EXTRACT(WEEK FROM CURRENT_DATE) SELECT DATEPART(isowk, GETDATE()) isowk for ISO 8601. DATEPART(wk, ...) returns US-style weeks anchored to January 1 - it will give the wrong week number in early January and late December.SELECT TO_CHAR(SYSDATE, 'IW') FROM DUAL IW = ISO-8601 week (1–52 or 1–53). WW = week starting Jan 1 (mostly unused).SELECT WEEK(NOW()) FROM sysibm.sysdummy1 [NSString stringWithFormat:@"Week %d",
[calendar ordinalityOfUnit:NSWeekCalendarUnit
inUnit:NSYearCalendarUnit
forDate:date]]; let gregorian = NSCalendar(calendarIdentifier:
NSCalendarIdentifierGregorian)!
gregorian.firstWeekday = 2 // Monday
gregorian.minimumDaysInFirstWeek = 4
let components = gregorian.components(
.WeekOfYearCalendarUnit | .YearForWeekOfYearCalendarUnit,
fromDate: date)
let week = components.weekOfYear lubridate::week() week_number = Time.now.strftime("%V") %U = week starting Sunday · %V = ISO-8601 · %W = week starting Monday.year, week := time.Now().ISOWeek() date +%V man date for more formats.Current_week = os.date("%V") Get-Date -UFormat %V
# or
"{0:d2}" -f ($(Get-Culture).Calendar.GetWeekOfYear(
$(Get-Date),
[System.Globalization.CalendarWeekRule]::FirstFourDayWeek,
[DayOfWeek]::Monday)) int weeknum;
weeknum = weekOfYear(today()); MESSAGE(FORMAT(CALCDATE('CW', TODAY), 0, '<week>')); Quiz: how well do you know ISO week numbers?
1. What does the "ISO" in ISO 8601 week numbering stand for?
2. Using the formula on this page, what dates does Week 34 of 2026 cover?
3. Which day of the week determines where Week 1 falls under ISO 8601?
4. How many years out of every 400-year Gregorian cycle have 53 ISO weeks?
5. In SQL Server, which DATEPART argument returns the correct ISO 8601 week number?