ISO Week Number Calculator
Find the ISO 8601 week number for a date, or look up the date range for a given ISO year and week — including whether that year has 52 or 53 weeks.
Date → ISO week number
ISO year + week → date range
Frequently Asked Questions
What is an ISO week number?
A standardized way (ISO 8601) to index weeks within a year: week 1 is the week — always Monday through Sunday — that contains the year's first Thursday, equivalently the week containing January 4. Every week has exactly 7 days and belongs to exactly one ISO week number.
Why does the ISO week sometimes belong to a different year than the calendar date?
Because ISO weeks always run Monday to Sunday and are assigned to whichever year owns the Thursday of that week. December 31, 2020 (a Thursday) is in ISO week 53 of 2020, and so is January 3, 2021 (the Sunday that ends that same week) — even though it's a January date in 2021.
Does every year have 52 weeks?
No — most years have 52 ISO weeks, but a year has 53 whenever January 1 falls on a Thursday, or the year is a leap year and January 1 falls on a Wednesday. 2020 and 2026 are both 53-week ISO years.
Where are ISO week numbers actually used?
Manufacturing date codes, payroll and shift scheduling, European business calendars, and ISO 8601 date strings themselves (format YYYY-Www-D) all commonly use ISO week numbers instead of, or alongside, the calendar month and day.