How the week number is calculated
Chronira uses the week numbering of the international standard ISO 8601:
- weeks run from Monday to Sunday;
- week 1 is the week containing the year’s first Thursday (equivalently, the week containing 4 January);
- a year has 52 or 53 weeks;
- the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next.
The current week number in the panel is calculated with your device’s own date. Below it you see the Monday-to-Sunday dates of the week, the day of the year and the days left in the year.
Find the week number of any date
Pick a date in Find the week number of any date and the tool answers at once: for example, 1 January 2026 is in week 1 of 2026. The full table of the current year lists every week with its Monday and Sunday, and the current week is highlighted.
Examples
| Date | Weekday | ISO week |
|---|---|---|
| 1 January 2021 | Friday | week 53 of 2020 |
| 30 December 2024 | Monday | week 1 of 2025 |
| 1 January 2026 | Thursday | week 1 of 2026 |
| 31 December 2026 | Thursday | week 53 of 2026 |
| 1 January 2027 | Friday | week 53 of 2026 |
2026 starts on a Thursday, so it has 53 ISO weeks: week 1 begins on Monday 29 December 2025, and week 53 runs until Sunday 3 January 2027. Week 1 of 2027 then begins on Monday 4 January 2027.
Where 1 January lands
| 1 January falls on | 1 January is in |
|---|---|
| Monday, Tuesday, Wednesday or Thursday | week 1 of the new year |
| Friday, Saturday or Sunday | the last week (52 or 53) of the previous year |
Common uses
- Planning sprints, shifts, deliveries or school terms by week number.
- Filling in forms or timesheets that ask for the calendar week.
- Checking which week a deadline or a holiday falls in.
Good to know
- Some calendars and spreadsheet functions count weeks differently, for example starting weeks on Sunday or always putting 1 January in week 1. Their numbers can differ from ISO weeks by one, especially at the start and end of the year.
- For a printable calendar with week numbers, see the 2026 calendar with week numbers and the 2027 calendar with week numbers.
- The date calculator also shows the ISO week of any date you reach by adding days, weeks or months.