Week Number
Find the current week number.
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About Week Number
Week Number figures out the ISO-8601 week number for any date. Pick a day (it starts on today) and it shows the ISO week, the ISO week-year, the day-of-year, and the exact Monday-to-Sunday dates that week covers. A reverse mode goes the other way: type a year and week number to get its date range.
It follows the real ISO rules, so weeks start on Monday and week 1 is the week holding the year's first Thursday, and it handles the awkward late-December and early-January boundaries correctly. It is completely free and runs entirely in your browser, so nothing you enter is ever uploaded.
Frequently asked questions
- Is this week number tool free to use?
- Yes, it is completely free with no sign-up, account, or usage limits. Look up as many dates and week numbers as you want.
- Is my data uploaded anywhere?
- No. Every calculation happens locally in your browser using JavaScript. The date you pick and any year or week you enter never leave your device and are never sent to a server.
- How is the ISO-8601 week number calculated?
- Under ISO-8601, weeks begin on Monday and week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4th. The tool assigns each date to the week owning its Thursday, which is why some early-January dates belong to the previous year's last week.
- Why does the week-year sometimes differ from the calendar year?
- Because ISO weeks never split across the Monday-Sunday boundary, the first days of January can fall in week 52 or 53 of the previous year, and the last days of December can fall in week 1 of the next year. The tool shows this ISO week-year separately and flags it when it differs from the calendar year.
- Can I convert a week number back into dates?
- Yes. Switch to the reverse mode, enter a year and a week number, and the tool shows that week's Monday start date and Sunday end date. It also tells you how many weeks that year has so you cannot enter an out-of-range week.
- Which years have 53 weeks instead of 52?
- A year has 53 ISO weeks when January 1st is a Thursday, or when it is a leap year that starts on a Wednesday. All other years have 52. The tool detects this automatically and displays the total week count for the year you are viewing.