Week Number Calculator
Under ISO 8601, weeks start on Monday and week 1 is the week with the year's first Thursday. Wondering what week it is right now? Enter a date — it defaults to today — to get its week number, week-year, and full Monday-to-Sunday range.
Week of
- This week runs
- Day of the week
The ISO 8601 rule
Three facts define the system: weeks begin on Monday, week 1 is the week containing the first Thursday of the year, and every date belongs to exactly one week-year. The week-year is usually the same as the calendar year — except for a few days around New Year, when a date can belong to the previous or next week-year.
Two examples worth remembering: December 30, 2024 (a Monday) is in week 1 of 2025, and January 1, 2027 (a Friday) is in week 53 of 2026. If your planning, payroll, or logistics system disagrees with a colleague's, this edge is almost always why.
Frequently asked questions
What is the ISO 8601 week numbering system?
It is the international standard for numbering weeks. Weeks start on Monday, and week 1 of a year is the week containing the first Thursday — equivalently, the week containing January 4th. Every week belongs to exactly one week-year, which is why December 30, 2024 is in week 1 of 2025, not week 53 of 2024.
Why is January 1 sometimes in week 52 or 53 of the previous year?
Because weeks start on Monday. If January 1 falls on Friday, Saturday, or Sunday, those days belong to the last week of the previous week-year. For example, January 1, 2027 (a Friday) is in week 53 of 2026. The week-year and the calendar year only differ on these edge days.
How is this different from week numbers in the US?
In common US usage, weeks start on Sunday and week 1 is simply the week containing January 1st — so week numbers can differ by one from ISO around year boundaries. This calculator uses the ISO 8601 standard, which is the norm in Europe and in most business and logistics contexts worldwide.
Can a year have 53 weeks?
Yes. A year has 53 ISO weeks when January 1 is a Thursday, or when it is a Wednesday in a leap year. Recent examples: 2020 and 2026 both have 53 weeks. Most years have 52.
How do I get the week number in Excel or Google Sheets?
Use =ISOWEEKNUM(date) in Excel (2013 and later) or Google Sheets — both implement ISO 8601, matching this calculator. The older =WEEKNUM(date) uses Sunday-start weeks and can disagree near January 1st.