How to use
- Choose Week number of a date, then enter a date. It starts as today.
- Or choose Dates of a week number, then enter a year and a week.
- Read the ISO week, its Monday-to-Sunday dates and the US-style week.
- Open Show every week of the year for the full list.
How ISO week numbering works
- Weeks run Monday to Sunday.
- Week 1 is the week that contains the year’s first Thursday. That is always the week with 4 January.
- A week belongs to the year its Thursday falls in. So 29–31 December can be in week 1 of the next year, and 1–3 January in week 52 or 53 of the year before.
- A year has 52 or 53 weeks. It has 53 when it starts on a Thursday, or is a leap year that starts on a Wednesday.
The ISO week date writes this as 2026-W43-3: ISO year, week, then the weekday (1 = Monday).
Examples
- 21 October 2026 is in week 43, from Monday 19 to Sunday 25 October 2026.
- 3 January 2021 is in week 53 of 2020.
- 30 December 2024 is in week 1 of 2025.
Week 1 by year
| ISO year | Week 1 starts | Weeks |
|---|---|---|
| 2024 | Mon 1 Jan 2024 | 52 |
| 2025 | Mon 30 Dec 2024 | 52 |
| 2026 | Mon 29 Dec 2025 | 53 |
| 2027 | Mon 4 Jan 2027 | 52 |
| 2028 | Mon 3 Jan 2028 | 52 |
| 2029 | Mon 1 Jan 2029 | 52 |
| 2030 | Mon 31 Dec 2029 | 52 |
ISO and US weeks compared
US-style weeks start on Sunday, and week 1 is the week with 1 January, so the first and last weeks can be short.
| Date | ISO week | US week |
|---|---|---|
| Thu 1 Jan 2026 | 1 | 1 |
| Sun 4 Jan 2026 | 1 | 2 |
| Wed 21 Oct 2026 | 43 | 43 |
| Thu 31 Dec 2026 | 53 | 53 |
| Fri 1 Jan 2027 | 53 (of 2026) | 1 |
Spreadsheet functions
In Excel, ISOWEEKNUM(date) gives the ISO week. WEEKNUM(date) gives the US-style week, with weeks starting on Sunday. WEEKNUM(date, 21) uses the ISO rule.
US-style numbering can reach week 54 in a leap year that starts on a Saturday, as in 2000.
Need the weekday of a date? Use the day of the week calculator. To move a date forward by weeks, use the date calculator.