๐ Week Number Calculator
Enter a date to see the calculation
Enter a date and instantly find its ISO 8601 week number (which week of the year it falls in) along with its day-of-year count. Useful for weekly business reports or checking week-number notation in a schedule.
How to use
- Enter the date you want to check.
- The ISO week number and day-of-year count are shown automatically.
How the calculation works
Enter a date to see its ISO 8601 week number (which week of the year it is) and its day of the year counted from January 1. ISO 8601 week rules โข Weeks start on Monday and end on Sunday โข Week 1 is the week containing the yearโs first Thursday (equivalently, the week containing January 4) โข A year has 52 or 53 weeks Because of these rules, a few days around New Year can belong to a week of the previous or next year. The result therefore shows the week-numbering year (ISO year), not the calendar year of the date. This numbering is common in European companies and in weekly reporting and production planning.
Worked example
Thursday, January 1, 2026 โ 2026 week 1 (day 1) Saturday, September 19, 2026 โ 2026 week 38 Thursday, December 31, 2026 โ 2026 week 53 Friday, January 1, 2027 โ 2026 week 53 (belongs to the previous year) Monday, January 4, 2027 โ 2027 week 1 Monday, December 30, 2024 โ 2025 week 1 (belongs to the next year)
Things to be aware of
- Week numbers can differ by one from the US-style system, where weeks start on Sunday and week 1 contains January 1.
- In ISO 8601 notation, Saturday of 2026 week 38 is written 2026-W38-6. The ISO Week Date Converter handles that format.
- The day of the year (January 1 = day 1) is one higher from March onward in leap years.
FAQ
What is an ISO week number?
A week number based on the ISO 8601 standard, where weeks start on Monday and week 1 is the week containing the year's first Thursday. Near year boundaries, the calendar year and the ISO week-numbering year can differ.
Does this support US-style (Sunday-start) week numbering?
No โ this tool only supports ISO 8601 (Monday-start) week numbering.
What is the day-of-year count?
The number of days that have elapsed since January 1st of that year, counting January 1st as day 1.