Week Number Calculator
The ISO 8601 standard defines week numbering used worldwide in business, logistics, and project management. Under this standard, weeks always begin on Monday, and week 1 is the week that contains the year's first Thursday — meaning a few days in early January may belong to week 52 or 53 of the previous year, and late-December days may fall in week 1 of the next year.
This calculator tells you the ISO week number for any date, the day of the year (ordinal date), and how many days remain in the calendar year. These figures are essential for scheduling, reporting cycles, and tracking annual progress at a glance.
How it works
ISO week number: shift the target date to the nearest Thursday (add 4 − weekday, where Monday = 1), then compute ⌈(dayOfYear(Thursday) / 7)⌉. The ISO week-year is the calendar year that contains that Thursday.
Use cases
- Identifying the correct billing or payroll week in enterprise software
- Planning project sprints and Gantt chart milestones by week number
- Tracking manufacturing or retail reporting periods (e.g., week 23 inventory)
- Verifying calendar week numbers printed on European and international planners
- Calculating how many days or weeks remain until year-end for annual goal planning
Frequently asked questions
How is the ISO week number calculated?
The ISO 8601 method shifts the target date to the Thursday of its week (weeks run Monday through Sunday), then divides that Thursday's day of the year by 7 and rounds up. Week 1 is always the week that contains the year's first Thursday. This is the numbering used in business software, logistics, and most European calendars.
How many weeks are in a year?
A calendar year contains 52 full weeks plus 1 or 2 extra days, so an ISO year has either 52 or 53 numbered weeks. A year gets 53 ISO weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year. Most years have exactly 52 ISO weeks.
Why can January 1 belong to week 52 or 53 of the previous year?
Under ISO 8601 every week belongs entirely to one week-year, and week 1 is the week containing the year's first Thursday. If January 1 falls on a Friday, Saturday, or Sunday, those first days still belong to week 52 or 53 of the previous ISO year. Conversely, December 29–31 can already fall in week 1 of the following year.
What day does the week start on in ISO 8601?
ISO 8601 weeks always start on Monday and end on Sunday, with days numbered Monday = 1 through Sunday = 7. This differs from the convention in the United States and Brazil, where calendars often display Sunday as the first day. Week numbers printed in European and international planners follow the Monday-start ISO rule.
What is the day of the year (ordinal date)?
The ordinal date is the sequential position of a day within its year, from 1 for January 1 up to 365 — or 366 in a leap year — for December 31. For example, March 1 is day 60 in a common year and day 61 in a leap year. This calculator shows it alongside the ISO week number and the days remaining in the year.