Week Number Calculator
Find ISO week numbers, week-year boundaries, weekday numbers, and week date ranges for planning dates.
Enter a calendar date to find its week number, week-year, weekday number, and date range without sending anything off this page. ISO weeks are the default because they are widely used for business planning, release schedules, education, and reporting.
Use the date you want to place on a week calendar.
Choose ISO weeks for standards-based week-year reporting, or a Jan 1 system for simpler wall-calendar planning.
Use the lookup fields when you know a week-year and week number and need the matching date range or weekday date.
For ISO and Monday-start weeks, 1 is Monday. For Sunday-start weeks, 1 is Sunday.
How it works
ISO 8601 week dates use Monday as the first day of the week. ISO week 1 is the week that contains January 4, which is also the week containing the first Thursday of the year. Because of that rule, dates near New Year can belong to the previous or next ISO week-year.
The Jan 1 options use a simpler calendar convention: week 1 is the partial or full week containing January 1, with either Sunday or Monday as the first day. These are useful for quick wall-calendar planning, but they are not ISO week dates.
The summary shows the selected system, week number, week-year, weekday number, week start and end dates, day of year, and how many weeks the selected week-year contains. All calculations happen locally in your browser.
