Week Number Calculator
Find week number of a date
Frequently Asked Questions
How do I find the week number of a date?
Enter any date and the calculator shows its ISO week number (1-53). ISO weeks start on Monday. Week 1 is the week containing the first Thursday of the year. January 1 may fall in week 52 or 53 of the previous year.
What is the ISO week numbering system?
ISO 8601 defines weeks starting Monday. Week 1 contains January 4 (or equivalently, the first Thursday). Most years have 52 weeks; years starting on Thursday (or Wednesday in leap years) have 53 weeks.
Why does January 1 sometimes show as week 52 or 53?
If January 1 falls on Friday, Saturday, or Sunday, it belongs to the last week of the previous year by ISO rules. For example, January 1, 2027 (Friday) is in week 53 of 2026. This ensures weeks are not split across years.
How are week numbers used in business?
Week numbers are common in European business planning, manufacturing schedules, project management, and reporting. "Delivery in W12" means the 12th week of the year (mid-March). They provide a compact date reference.
How do I convert a week number back to a date range?
Enter a year and week number to get the Monday-Sunday date range. Week 10 of 2025 = March 3-9. The calculator shows the full date range for any ISO week number in any year.