What Week Is It? ISO Week Number Today

How to use it

The big number at the top is the current ISO week, worked out from your device’s date and time zone. It updates by itself if you leave the page open past midnight.

• Week of a date: pick any date to get its ISO week, its full ISO week date (such as 2026-W39-4, meaning the Thursday of week 39), the day of the year and the US-style week number.
• Dates of a week: enter a year and a week number to see which Monday and Sunday it runs between.
• All weeks in a year: the table lists every week with its first and last day and highlights the current week. Switch the numbering to US if your calendar starts on Sunday.
• Downloads: save the year as CSV for a spreadsheet or project plan, or as an .ics calendar file with one all-day event per week, named “Week 39”, either on the week’s first day or spanning the whole week. Google Calendar, Outlook and Apple Calendar all import .ics files. Import it into a separate calendar created just for week numbers, so you can hide or remove them all at once.

How ISO 8601 week numbers work

ISO 8601 defines three rules. Weeks start on Monday. Week 1 is the week that contains the first Thursday of the year, which is the same as the week that contains January 4. And every week belongs entirely to the year that holds its Thursday, called the week-year.

Because of that last rule, the days around New Year can belong to a neighbouring year’s week. December 29 to 31 can fall in week 1 of the next year, and January 1 to 3 can fall in week 52 or 53 of the previous one. Always pair an ISO week with its week-year, not the calendar year.

Years with 53 weeks

Most years have 52 ISO weeks. A year has 53 when January 1 is a Thursday, or when it is a leap year that starts on a Wednesday. That happens in 71 of every 400 years; 2020 and 2026 are recent examples, and the next one after 2026 is 2032. December 28 is always in the last week of its year, which gives a quick way to count them.

US week numbers (not ISO)

Many US calendars count weeks from Sunday and call the week containing January 1 week 1, however few days of it fall in the new year. The first and last weeks are often partial, so a year has 53 weeks and occasionally 54 (2028 does, because it is a leap year that starts on a Saturday). This is what WEEKNUM(date) returns in Excel and Google Sheets. The ISO number comes from ISOWEEKNUM(date) instead.

Getting ISO weeks in code

• Python: date.isocalendar() returns the week-year, week and weekday.
• strftime: use %G-W%V. Pairing %V with %Y gives the wrong year at the turn of the year, and %U is a Sunday-based count that starts at week 0.
• PostgreSQL: EXTRACT(WEEK FROM d) and EXTRACT(ISOYEAR FROM d).
• MySQL: WEEK(d, 3) is the ISO week.

This page does its date math on plain year, month and day numbers rather than on clock times, so daylight saving changes can never shift a date into the wrong week.