Week Number Calculator
Find the ISO 8601 week number for any date.
📐 ISO 8601: weeks start on Monday; week 1 is the week containing the year's first Thursday.
The week number calculator tells you which week of the year any date falls in, following either the ISO 8601 standard (used in Europe and most international business) or the US/simple standard. It also finds the start and end dates of any week number, and helps with scheduling, project planning, and payroll cycles that operate on weekly periods.
📋 How to Use This Calculator
Enter any date to find its week number under both ISO 8601 (weeks start Monday, week 1 = first week with a Thursday) and US convention (weeks start Sunday, week 1 = week containing January 1). Or enter a year and week number to find the Monday–Sunday dates for that week. The calendar view shows the full year with week numbers.
💡 Key Facts & Information
ISO 8601 week numbering: Week 1 of the year is the week containing the first Thursday of January. Weeks run Monday to Sunday. A year has 52 or 53 ISO weeks. January 1 can fall in week 52 or 53 of the previous year if it is a Monday, Tuesday, or Wednesday. December 31 can fall in week 1 of the next year if it is a Thursday, Friday, or Saturday. US week numbering: week 1 starts on January 1 (partial week); weeks run Sunday to Saturday. Business applications: many B2B contracts specify delivery by "week 15" or "Q2 week 3" — always clarify ISO or US convention. Indian financial year runs April–March; businesses tracking weekly sales should align week numbers to their fiscal year.