ISO Week Calculator
Convert ISO year + week to start (Monday) and end (Sunday) dates.
Enter a year and an ISO 8601 week number (1-53) and instantly see the Monday-to-Sunday range of that week, plus a mini calendar of all seven days. Validates that the week exists in that ISO year.
Common use cases
- Decode a label like "2026-W19" into actual calendar dates
- Schedule sprints that run on ISO week boundaries
- Coordinate with European partners using ISO weeks
- Verify whether a year has a week 53
Frequently asked questions
Why is week 1 sometimes in late December?▾
ISO 8601 anchors week 1 on the year's first Thursday. If Jan 1 falls late in the week, it can belong to week 52 or 53 of the previous year, and the new year's week 1 starts the following Monday.
Does my year have 53 weeks?▾
ISO years have 53 weeks if Jan 1 is a Thursday, or it's a leap year and Jan 1 is a Wednesday. The tool tells you the maximum valid week for the chosen year.
Are weeks Monday-Sunday or Sunday-Saturday?▾
ISO 8601 weeks always run Monday to Sunday, regardless of locale.
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