Week numbers explained
Week numbers are widely used for training plans, production schedules, school terms and project planning. This tool follows ISO 8601, the international standard: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year.
Because of that rule, the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next year. A year has either 52 or 53 ISO weeks.
How to use the tool
- The current week number is shown at the top with its Monday and Sunday.
- Pick any date to see which week it falls in.
- Enter a week and a year to see its start and end dates.
- Browse the table to see every week of the year; use the arrows to change year.
Frequently asked questions
Why does January 1 sometimes show week 52 or 53?
Under ISO 8601, week 1 is the week with the year’s first Thursday. If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.
Does every country use ISO week numbers?
Most of Europe and much of Asia do. In the United States, calendars sometimes count weeks starting on Sunday, which can give a different number.