ISO Week Number Calculator
Find the ISO 8601 week number for any date, or look up week date ranges.
Week 22
ISO Week Number
2026
ISO Week Year
Mon, May 25, 2026
Week Start (Monday)
Sun, May 31, 2026
Week End (Sunday)
Week Number → Date Range
About This Tool
The ISO Week Number Calculator implements the ISO 8601 standard for week numbering. ISO weeks start on Monday and the first week of the year is the week that contains the year's first Thursday. This means week 1 of a year may start in December of the previous year, and the last days of December may belong to week 1 of the next year.
How to Use
- Select a date to find its ISO week number and week date range.
- Or enter a year and week number to find the corresponding Monday–Sunday range.
Use Cases
Businesses that report on a weekly cadence use ISO week numbers for consistent reporting. Developers implementing scheduling systems need to convert between week numbers and calendar dates. Supply chain teams reference purchase orders by ISO week.
FAQ
- Why might week 1 start in late December? — ISO 8601 week 1 is defined as the week containing January 4th. If January 1 is a Friday, Saturday, or Sunday, that week is already the last week of the previous year.
- Can a year have 53 weeks? — Yes, years that start on Thursday, or leap years starting on Wednesday, have 53 ISO weeks.