How to use
The current week number is shown by default. Pick any date to see its week number and the first and last day of that week.
ISO 8601 vs. the US system
In the US, weeks start on Sunday and week 1 is the week that contains January 1.
Most of Europe and many international organizations use ISO 8601:
- weeks start on Monday;
- week 1 is the week that contains the first Thursday of January.
Because of this, the last days of December can belong to week 1 of the next year, and the first days of January to the last week of the previous year. For example, January 1, 2027 is in week 53 of 2026 under ISO 8601. Week numbers in the two systems can differ by one.
Example
October 19, 2026 is in week 43 under both systems: Monday, October 19 to Sunday, October 25 in ISO 8601, and Sunday, October 18 to Saturday, October 24 in the US system.
FAQ
How many weeks are in a year? Under ISO 8601, 52 or 53. 2026 has 53 ISO weeks.