Week number
Find the ISO week number of a date, or the dates of a week. ISO 8601 weeks start on Monday, and week 1 is the week with the year's first Thursday.
Week number
todayDates of a week
How it works
ISO 8601 numbers weeks from Monday to Sunday. Week 1 is the week that holds the year's first Thursday, which is the same as the week holding 4 January. So 1 to 3 January can belong to the last week of the year before, and 29 to 31 December to week 1 of the next year. Most of Europe and Asia count weeks this way.
Most years have 52 ISO weeks. A year has 53 when it starts on a Thursday, or is a leap year starting on a Wednesday. 2026 has 53; the next after that is 2032.
In the US, weeks usually start on Sunday and week 1 is the week that holds 1 January, however short. That's Excel's WEEKNUM(date). The two systems often differ by one, so say which one you mean. Excel's ISOWEEKNUM(date) gives the ISO week.
To add days or weeks to a date, use the date calculator. To count the days between two dates, use the time difference calculator.