ISO Week Number
What ISO 8601 week is any date in?
ISO 8601 week
2026-W19
Week 19 of 53 · Sunday
Week starts: 2026-05-04 (Monday)
Week ends: 2026-05-10 (Sunday)
Day of week: 7 (1 = Mon, 7 = Sun)
ISO 8601 weeks always start on Monday. Week 1 is the week containing the year's first Thursday, which means a January 1 can fall in week 52 or 53 of the previous year. Most years have 52 weeks; some have 53.
About
Enter any date. Get the ISO 8601 week number in YYYY-Www format, plus the day of week (1–7), the Monday and Sunday bookends of that week, and how many ISO weeks the year contains.
How to use
- Pick a date.
- Read the ISO week number.
FAQ
Why does January 1 sometimes fall in week 52 or 53?+
ISO weeks always start on Monday and week 1 is the week containing the first Thursday. So if January 1 is a Friday or later, it ends up in the previous year's last week.
Why are there sometimes 53 weeks?+
Years that start on Thursday (or leap years that start on Wednesday) have 53 ISO weeks. About 71 of every 400 years.