ISO week number calculator
ISO 8601 weeks start on Monday, and the year's first week is the one containing its first Thursday. A date in early January can therefore belong to the last week of the previous year.
- Week starts (Monday)
- 31/08/2026
- Week ends (Sunday)
- 06/09/2026
Rates and thresholds
- Standard
- ISO 8601
- First day of the week
- Monday
- The year's first week
- Contains the first Thursday, i.e. 4 January
- Weeks in a year
- 52 or 53
- Written form
- 2026-W07
Why is 1 January sometimes in week 52 or 53 of the previous year?
Because an ISO week is never split across the year boundary — each week belongs wholly to one year. The first week is the one holding the first Thursday, so when 1 January falls on a Friday, Saturday or Sunday it still belongs to the previous year's last week. The same rule puts 31 December into the next year's first week in some years.
Does every country number weeks the same way?
No, and it is a common source of confusion in international work. Estonia and Europe use ISO 8601, where the week starts on Monday. In the United States the week starts on Sunday and week 1 is the one containing 1 January, which often gives a number one different for the same date. This calculator uses the ISO rule.