Find the ISO 8601 week number for any date and see the start and end dates of that week.
About ISO weeks
ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. The ISO year can differ from the calendar year near January 1.
Result
Saturday, 21 March 2026 falls in ISO week 12 of 2026.
Week range
Mon, 16 Mar 2026 to Sun, 22 Mar 2026
Also in Calendar Reference
Dates & Time
Find the ISO 8601 week number for any date, along with the start and end dates of that week.
The ISO 8601 standard assigns every date to a numbered week (1–53). Weeks start on Monday and the first week of the year is the one containing the first Thursday of January. This means dates near the new year boundary can belong to a different ISO year than their calendar year.
Businesses, project managers, and logistics teams use ISO week numbers for consistent scheduling across countries. Unlike calendar months, ISO weeks always span exactly 7 days, making comparisons and planning simpler.
Frequently asked questions
Yes — if January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last ISO week (52 or 53) of the previous year.
Most years have 52 ISO weeks, but years that start on a Thursday (or leap years starting on a Wednesday) have 53 weeks.
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