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