Week Number Calculator
Pick any date and find out its ISO week number - the calendar week it falls in. Also shows the Monday-to-Sunday range for that week. Useful if you work with weekly reporting, sprints, or anything that references week numbers.
Week starts (Monday)
Monday, 20 Jul 2026
Week ends (Sunday)
Sunday, 26 Jul 2026
How to use
- Pick any date using the date picker.
- The week number updates instantly - no button needed.
- ISO weeks start on Monday, and Week 1 is the one that contains the year's first Thursday.
Related tools:
ISO week numbering starts on Monday and the first week of the year is the one containing the first Thursday. This means January 1 can fall in week 52 or 53 of the previous year, and December 31 can fall in week 1 of the next year - which confuses people who just want to know 'what week number is this date'.
Week numbers are used in project management (sprint planning, reporting periods), manufacturing and supply chain (production scheduling), and sports (fixture scheduling). If someone says a meeting is in Week 37, this tool tells you exactly what dates that covers.
Frequently Asked Questions
Why does January 1 sometimes fall in week 53 of the previous year?
ISO week 1 is defined as the week containing the first Thursday of the year. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year. So December 30, 2024 might be in Week 1 of 2025 by ISO numbering.
Does every year have 52 weeks?
Most years have 52 weeks plus 1 or 2 extra days. Some years have 53 ISO weeks - this happens when January 1 is a Thursday, or in leap years when January 1 is a Wednesday. The calculator shows the total ISO weeks for any year.
Are week numbers the same in all countries?
ISO 8601 (weeks start Monday, first week contains first Thursday) is international standard. The US sometimes uses a different convention where weeks start on Sunday and week 1 begins January 1 regardless of day. Clarify which standard applies when working internationally.