What week number is it today?
The current week number is shown at the top with its Monday and Sunday. Below, turn a date into a week number or a week number into dates, and export the whole year as a calendar file.
Source: IEEE Std 1003.1 (POSIX) – strftime(), %V: ISO 8601 week number. Updated: .
How it is calculated
How week numbers are counted
Most of the world, including Europe and business software, uses ISO 8601: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year – which is the same as the week containing January 4. That is why January 1 can still belong to week 52 or 53 of the previous year, and December 29–31 can already be week 1 of the next year.
A year has 52 or 53 ISO weeks. It has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year – as in 2020 and 2026.
US week numbers
Many US calendars start the week on Sunday and call the week containing January 1 “week 1” – the default of Excel’s WEEKNUM function (System 1). The two counts usually match but can differ by one week around New Year, so this page shows both. A US-style year can even have 54 partial weeks, as in 2028, which starts on a Saturday and ends on a Sunday.
Example
September 28, 2026 is a Monday. The Thursday of that week is October 1, day 274 of the year. (274 − 1) ÷ 7 = 39 remainder 0, plus 1 gives ISO week 40, Monday, September 28 to Sunday, October 4, 2026.
Week numbers by year
Overviews with each year’s quirks: week numbers 2026, week numbers 2027, week numbers 2028. The table above switches to any year from 2020 to 2035, prints cleanly and exports an ICS file for Outlook, Google Calendar or Apple Calendar, with one all-day entry per week from Monday to Sunday.
Limits
“Today” is based on your device’s date. Some regions count weeks from Saturday; those systems are not shown here.
Frequently asked questions
What week number is it today?
The current week is shown in large type at the top of this page, with its Monday and Sunday. It is calculated per ISO 8601 from your device’s date; the US-style number is shown right below.
How many weeks are in 2026?
53 ISO weeks. January 1, 2026 is a Thursday, which gives the year a 53rd week, running from December 28, 2026 to January 3, 2027. 2027 and 2028 have 52 ISO weeks each.
Why does my calendar show a different week number?
US-style calendars start on Sunday and count the week of January 1 as week 1. In Outlook, set the first day of the week to Monday and the first week of the year to “First 4-day week” to get ISO week numbers.
How do I get the week number in Excel?
Use =ISOWEEKNUM(A1) or =WEEKNUM(A1,21) for ISO weeks. Plain =WEEKNUM(A1) uses the US system (week starts Sunday) and can differ at the start of the year.
When does week 1 start?
On the Monday of the week that contains January 4. In 2026 that was December 29, 2025; in 2027 it is January 4, 2027.
Sources and legal basis
- IEEE Std 1003.1 (POSIX) – strftime(), %V: ISO 8601 week number
- Microsoft Support: WEEKNUM function (System 1 = US, System 2 = ISO 8601)
- Wikipedia: ISO week date
- IETF RFC 5545 – iCalendar (ICS file format)
As of:
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