Week Number Calculator
Many businesses, project plans and software systems refer to dates by their week number rather than the day of the month. The international standard, ISO 8601, defines week 1 as the week containing the first Thursday of the year, with weeks starting on Monday. This calculator takes a date and returns its ISO 8601 week number, the ISO week-year (which can differ from the calendar year near January), and the day of the year. The rule is a precise definition, so the result is exact, with no estimation.
ISO 8601 week number rule
Weeks start on Monday (Monday = 1 ... Sunday = 7)
Week 1 = the week containing the year's first Thursday
Take the Thursday of the target date's week
ISO week-year = the year of that Thursday
Week number = 1 + floor((that Thursday - first Thursday) / 7 days)
The algorithm finds the Thursday in the same ISO week as the input date, because every ISO week is owned by the year of its Thursday. The week number counts whole weeks from the year's first Thursday.
Week number context
- ISO 8601 weeks always begin on Monday and end on Sunday.
- Week 1 contains the first Thursday and equivalently the date 4 January.
- Early January dates can fall in week 52 or 53 of the previous week-year.
- An ISO year has 53 weeks when it starts on a Thursday (or a leap year starting Wednesday).
- The notation week-year-Wweek, such as 2026-W25, identifies a week unambiguously.
Week numbers: frequently asked questions
How is the ISO 8601 week number defined?
Under ISO 8601, weeks start on Monday and week 1 is the week containing the first Thursday of the year, which is equivalently the week containing 4 January. Weeks are numbered 1 to 52 or 53. This is the standard used across Europe and in business and software internationally.
Why can a date in early January be in week 52 or 53?
Because ISO week 1 must contain the year's first Thursday, the first few days of January can belong to the last week of the previous year. For example, 1 January 2022 fell on a Saturday and belongs to ISO week 52 of week-year 2021. The calculator reports the ISO week-year so this is unambiguous.
What is the ISO week-year?
The ISO week-year is the year that owns a given ISO week, which can differ from the calendar year for dates near 1 January or 31 December. A date is written as week-year-Wweek, such as 2026-W25. The week-year keeps weeks whole rather than splitting them across a calendar boundary.
How many weeks are in an ISO year?
Most ISO years have 52 weeks. A year has 53 weeks when it starts on a Thursday, or is a leap year that starts on a Wednesday. This recurs roughly every five to six years, which is why some calendars show a week 53.
Is the ISO week number the same as the US week number?
Not always. Some US calendars number weeks with Sunday as the first day and treat the week containing 1 January as week 1, without the first-Thursday rule. This calculator uses the ISO 8601 definition, which is the international standard and the one most software uses.
Official sources
- International Organization for Standardization: ISO 8601 date and time format.
- U.S. National Institute of Standards and Technology: Time and Frequency Division.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.