Week Number Calculator
Find the ISO 8601 week number (1–53) for any date.
What is ISO Week Number?
ISO 8601 defines week 1 as the week containing the year's first Thursday (equivalently: the week containing January 4). Weeks start on Monday. This means Dec 29–31 may belong to week 1 of the next year, and Jan 1–3 may belong to week 52 or 53 of the previous year.
Week 53
Most years have 52 weeks. A year has week 53 if it starts on Thursday, or is a leap year starting on Wednesday. Recent years with week 53: 2004, 2009, 2015, 2020, 2026. The next: 2032.
Where Week Numbers Are Used
Business planning
Sprint planning, quarterly reporting in Europe
Manufacturing
Production schedules, lot numbering
EU/Scandinavian calendars
Week numbers printed on all calendars
ISO 8601 week date
YYYY-Www-D format for unambiguous dates