Use either method: date → week number, or (year + week) → Monday date.
ISO 8601: weeks start on Monday. Week 1 is the first week with at least 4 days in the new year.
Date ↔ ISO week number
Convert a calendar date to its ISO 8601 week number, or get the Monday date for a given ISO week.
ISO week numbers explained
ISO 8601 week numbers are widely used in business planning, logistics, project management and reporting. This tool lets you quickly find the ISO week for any date and retrieve the Monday starting date for an ISO week.
- Weeks start on Monday.
- Week 1 is the first week with at least 4 days in January.
- Some years include a week 53.