Week Number Checker — Find ISO Week Number for Any Date

ISO 8601 defines a week as starting on Monday and ending on Sunday. Week 1 of a year is defined as the week containing the year's first Thursday. This rule means that up to three days at the start of January can belong to the previous year's last week, and up to three days at the end of December can belong to the next year's week 1. For example, January 1, 2023 was a Sunday — it belonged to week 52 of 2022 because the Thursday of that week (December 29, 2022) was in 2022. Conversely, January 1, 2024 was a Monday — it belonged to week 1 of 2024 because the first Thursday of 2024 was January 4. This can result in years having either 52 or 53 weeks. Years with 53 ISO weeks occur when January 1 falls on a Thursday, or when it is a leap year and January 1 falls on a Wednesday.

Not all countries use ISO 8601 week numbering. The United States, Canada, and some other countries traditionally start the week on Sunday, which produces different week numbers than the ISO system. In the US system, the first week of the year is simply the week containing January 1 — so January 1 is always in week 1, and the week may be only 1 day long. This difference means that the same date can have different week numbers depending on the system used. Software systems often differ: MySQL's WEEK() function defaults to a Sunday-based system unless you specify mode 3 for ISO 8601. Python's datetime.isocalendar() returns ISO week numbers. JavaScript's Date object does not have built-in ISO week number support — it must be calculated manually. When sharing week numbers across international teams, always specify which system you are using.

The day of year (ordinal date) counts January 1 as day 1 and increments sequentially through the year. In a regular year, December 31 is day 365; in a leap year it is day 366. Day of year numbers are used in meteorology, astronomy, and some financial calculations. The fiscal quarter assignments in this tool follow the calendar year: Q1 is January–March, Q2 is April–June, Q3 is July–September, and Q4 is October–December. Note that many businesses use fiscal years that do not align with the calendar year — for example, Japan's fiscal year runs April 1 to March 31, and the US federal government fiscal year runs October 1 to September 30. This tool uses calendar year quarters; adjust manually if your organization uses a non-calendar fiscal year.

Week numbers simplify scheduling in industries with regular weekly cycles. Manufacturing and logistics use week numbers for production planning: "deliver by W14" means the 14th ISO week. Retail businesses use week numbers to align sales periods and inventory cycles. In project management, sprint schedules are often expressed in week numbers — "Sprint 22 starts in W18". When collaborating internationally, week numbers reduce ambiguity compared to date ranges: "W22" (the week of May 27, 2026 in ISO numbering) is unambiguous once both parties agree on the ISO system, whereas "end of May" is vague. European businesses, particularly in Germany, the Netherlands, and Scandinavia, heavily use ISO week numbers in daily business communication.

Leap years occur every four years (years divisible by 4), with exceptions for century years not divisible by 400. A leap year adds one day to February, making it 29 days and the year 366 days total. This affects the day of year count: in a leap year, March 1 is day 61 (instead of 60), and December 31 is day 366. Leap years can also affect whether a year contains 53 ISO weeks: if January 1 of a leap year falls on Wednesday, or January 1 falls on Thursday in any year, that year has 53 ISO weeks. This checker automatically detects whether the entered date falls in a leap year and displays the correct day-of-year values accordingly. Leap year detection is critical for date arithmetic — adding 365 days to a date within a leap year gives the wrong result by one day.