OmniCalcsWeek Number Calculator
Find the ISO week number of any date — or jump from a week number to its start and end dates.
How to Use This Calculator
Select any date to see its ISO week number and the Monday-to-Sunday range of that week.
- The ISO year can differ from the calendar year for dates in early January and late December.
- Many project-management tools and school calendars number weeks this way.
Frequently Asked Questions
What is an ISO week number?
The International Standard ISO 8601 numbering of weeks. Week 1 is the week containing the year's first Thursday, and weeks run Monday to Sunday.
Why is December 29 sometimes in week 1 of next year?
If the last week of the year has 4 or more days in the new year, those days belong to the new year's week 1.
Can a year have 53 weeks?
Yes, about once every 5-6 years, when January 1 falls on a Thursday (in a common year) or Wednesday (leap year).
Does the week start on Sunday?
In the ISO system, weeks start on Monday. Some regions use Sunday-based numbering — this tool follows ISO.
Practical Example
January 1, 2026 is a Thursday, so it belongs to ISO week 1 — the week running from Monday, December 29, 2025 through Sunday, January 4, 2026. Because January 1 falls on a Thursday in a common year, 2026 has 53 ISO weeks, and Thursday, December 31, 2026 falls in week 53.
- Enter January 5, 2026 to confirm the shift into week 2, the Monday after week 1 ends.
What Your Results Mean
- ISO week number — the date's position in the ISO 8601 Monday-to-Sunday week numbering.
- Week begins / ends — the Monday and Sunday that frame the week containing your date.
- ISO year — the year the week belongs to, which can differ from the calendar year for dates in early January and late December.