Week Number Calculator
Enter any date and find its ISO week number, plus the Monday-to-Sunday dates that week covers.
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This tool returns the ISO 8601 week number for any date, along with the start and end of that week, the day of year, and how far through the year you are.
Week numbers come up constantly in payroll, project scheduling, reporting, and sprint planning. The ISO standard is used by most business software and spreadsheet tools, with weeks running Monday to Sunday and the first week of the year defined as the one containing the first Thursday.
How ISO week numbers work
ISO week 1 is the week containing the first Thursday of the year, which means it always includes January 4th. Weeks run Monday to Sunday. Because of this, January 1st can fall in week 52 or 53 of the previous year, and the last days of December can be in week 1 of the next year. The tool shows the ISO year alongside the week number when it differs from the calendar year.
How it's worked out
- Weeks run from Monday to Sunday.
- Week 1 is the week that contains the first Thursday of the year, which is the same as the week that contains 4 January.
- Days before week 1 belong to the last week of the year before, and the last days of December can belong to week 1 of the next year.
- A year has 52 weeks, or 53 when 1 January is a Thursday, or a Wednesday in a leap year.
1 January 2026 is a Thursday, so it is in week 1, which starts on Monday 29 December 2025. That makes 2026 a 53-week year.
ISO weeks by year
| Year | Weeks in the year | Week 1 starts on |
|---|---|---|
| 2024 | 52 | Monday 1 January 2024 |
| 2025 | 52 | Monday 30 December 2024 |
| 2026 | 53 | Monday 29 December 2025 |
| 2027 | 52 | Monday 4 January 2027 |
| 2028 | 52 | Monday 3 January 2028 |
| 2029 | 52 | Monday 1 January 2029 |
| 2030 | 52 | Monday 31 December 2029 |
| 2031 | 52 | Monday 30 December 2030 |
| 2032 | 53 | Monday 29 December 2031 |
ISO weeks run Monday to Sunday. Week 1 is the week with the first Thursday of the year in it, so it can start in the last days of December.
Common mistakes
- Assuming week 1 starts on 1 January. ISO weeks start on a Monday, and week 1 is the one with the first Thursday.
- Using a different system than the people you are working with. The US often counts weeks differently.
- Forgetting that the last days of December can fall in week 1 of the next year, and the first days of January in week 52 or 53 of the old one.
Questions
Why does the year shown differ from the calendar year?
For dates at the very start or end of January and December, the ISO week can belong to a different ISO year than the calendar year. For example, January 1 2016 was in ISO week 53 of 2015. This is correct behaviour, not a bug.
Do weeks always start on Monday?
In ISO 8601, yes. Some countries and organisations use Sunday as the start of the week. If you need Sunday-based week numbers, the ISO figure will typically be one less than what you expect.
How is week 1 decided?
Under the ISO system, week 1 is the week with the first Thursday of the year in it. Weeks run Monday to Sunday.
Can a year have 53 weeks?
Yes. Some years have 52 ISO weeks and some have 53, which is why late December can fall in week 1 of the next year.
Another week, another research session.