How to find the week number
The calculator opens with today's date, so the result at the top is the current week number. Pick any other date to find its week. You will also see the date range of that week, the day of the year and a list of every week in the year.
ISO week numbers
Most of the world — and most business software, including Excel's ISOWEEKNUM — uses the ISO 8601 standard:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year's first Thursday — equivalently, the week that contains January 4.
- A year has 52 or 53 weeks. Years that start on a Thursday (and leap years that start on a Wednesday) have 53.
Because of these rules, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, January 1, 2021 was in ISO week 53 of 2020.
US week numbers
In the United States, calendars often start weeks on Sunday and count the week containing January 1 as week 1, however few days it has. This system can give a different number from ISO, usually by one, so check which convention your company or school uses.
How to calculate the ISO week number
Example: October 22, 2026 is a Thursday (weekday 4) and the 295th day of the year: (295 − 4 + 10) ÷ 7 = 43, so it falls in week 43. If the result is 0, the date belongs to the last week of the previous year; if it is 53 in a 52-week year, it belongs to week 1 of the next year.
Who uses week numbers?
Week numbers are common in Europe and Asia for school and work schedules, in manufacturing and retail for planning and reporting, in project management sprints and in payroll. Saying "week 43" avoids confusion about where a week starts and ends.
Frequently asked questions
What week of the year is it?
The calculator at the top of this page shows the current ISO week number based on your device's date, along with the Monday-to-Sunday date range of this week.
How many weeks are in a year?
A year has 52 weeks plus 1 day (2 in a leap year). In the ISO system, most years have 52 numbered weeks and some have 53.
When does week 1 start?
In the ISO system, week 1 is the week with the first Thursday of the year, so it starts on the Monday between December 29 and January 4. In the US system, week 1 is the week containing January 1.
Does 2026 have 53 weeks?
Yes. 2026 starts on a Thursday, so it has 53 ISO weeks. Week 53 runs from December 28, 2026 to January 3, 2027.
How do I get the week number in Excel?
Use =ISOWEEKNUM(A1) for ISO week numbers, or =WEEKNUM(A1) for the US system where weeks start on Sunday.
Results are provided for convenience and rounded for display. Sales tax rules, tipping customs and date conventions differ by location, so check local rules when it matters.
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