Week Number

What week is it? See the current ISO-8601 week number at a glance, or look up the week number, week of year and day of year for any date.

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Calculating the current week…
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Understanding ISO week numbers

Week numbers are widely used in business, manufacturing, logistics and project planning — "we ship in week 42" is a common shorthand. The international standard for them is ISO-8601, and it has two firm rules: weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week that includes January 4).

A consequence of those rules is that the first few days of January can belong to the last week (52 or 53) of the previous year, and the last days of December can belong to week 1 of the next year. That's why the tool shows the ISO week's own year next to the number. Most years have 52 ISO weeks; some have 53.

The lookup above also reports the week of year (a simple count of seven-day blocks from January 1) and the day of year (1 to 365 or 366), which are handy for spreadsheets and data work. To measure a gap between two dates instead, use the date calculator.

Frequently Asked Questions

What week number is it right now?
The banner at the top shows the current ISO-8601 week number, calculated from your device's date. ISO weeks start on Monday, and week 1 is the week containing the first Thursday of the year.
What is the ISO-8601 week numbering system?
It's the international standard for week numbers. Weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday (equivalently, the week with January 4). A year has either 52 or 53 ISO weeks.
Why does the week number sometimes belong to a different year?
Because ISO weeks don't split across the Thursday boundary. January 1 can fall in week 52 or 53 of the previous ISO year, and December 31 can fall in week 1 of the next. The tool shows the ISO week's own year to make this clear.
What's the difference between week of year and ISO week?
The "week of year" simply counts seven-day blocks from January 1, while the ISO week follows the Monday-start, first-Thursday rule used worldwide. They often match but can differ by one, especially around New Year.