ISO week number
What week of the year is it? Find the ISO 8601 week number for today or any date.
What is an ISO week number?
ISO 8601 defines a standard way to number weeks in a year. Under this standard:
- Weeks run Monday to Sunday
- Week 1 is the week containing the first Thursday of the year (equivalently, the week with January 4th)
- A year has either 52 or 53 ISO weeks
- The ISO year can differ from the calendar year — days at the very start or end of January may belong to the previous or next ISO year
Why does week 1 sometimes start in December?
If January 1st falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous ISO year — meaning week 1 of the new ISO year starts on the Monday before New Year's Day. Similarly, the last few days of December can be in ISO week 1 of the following year.
Who uses ISO week numbers?
ISO week numbers are common in European business, manufacturing, and logistics. Sprint planning in software teams often references week numbers. Financial reporting, academic calendars, and broadcast schedules in many countries are structured around ISO weeks.
Weeks in a year
Most years have 52 ISO weeks. A year has 53 weeks when January 1st is a Thursday, or (in a leap year) when it's a Wednesday or Thursday. The next 53-week years after 2026 are 2032 and 2037.
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