Week number calculator — ISO 8601 week of the year
Find the ISO 8601 week number for any date, look up the exact date range of a week number, and see the current week with the full year calendar.
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Loading the current week…
What the tool does
- Date to week — the ISO 8601 week number for any date, plus the week's Monday and Sunday.
- Week to dates — enter a week number and year and get all seven dates.
- Whole year — every week of a year in one scrollable table, with the current week marked.
The ISO 8601 rule
ISO 8601 defines week 1 of a year as the week containing the first Thursday of January. Three equivalent statements of the same rule:
- Week 1 is the week containing 4 January.
- Week 1 is the first week with at least four days in the new year.
- Week 1 is the week containing the first Thursday of January.
Weeks always run Monday to Sunday, and a week belongs entirely to the year that holds its Thursday. That last clause is why 30 December can be in week 1 of the following year, and why 1 January can be in week 52 or 53 of the previous one. The tool calls this the ISO week-numbering year and shows it whenever it differs from the calendar year.
52 or 53 weeks?
A 365-day year is 52 weeks plus one day; a leap year is 52 weeks plus two. Those spare days accumulate, and every five or six years a 53rd week appears. Precisely: a year has 53 ISO weeks when 1 January is a Thursday, or when it is a Wednesday and the year is a leap year.
| Year | 1 January falls on | ISO weeks |
|---|---|---|
| 2024 | Monday (leap year) | 52 |
| 2025 | Wednesday | 52 |
| 2026 | Thursday | 53 |
| 2027 | Friday | 52 |
| 2028 | Saturday (leap year) | 52 |
| 2029 | Monday | 52 |
| 2030 | Tuesday | 52 |
| 2031 | Wednesday | 52 |
| 2032 | Thursday (leap year) | 53 |
The 53-week year is a genuine problem in retail and payroll: annual comparisons break, and weekly-paid staff receive an extra pay run. Systems built on a 52-week assumption tend to fail in exactly those years.
ISO weeks versus US weeks
| ISO 8601 | Common US / broadcast | |
|---|---|---|
| Week starts | Monday | Sunday |
| Week 1 is | The week containing 4 January | The week containing 1 January |
| Partial first week | Never — belongs to the previous year | Yes — can be one day long |
| Weeks per year | 52 or 53 | 52 or 53 |
| Used by | Europe, ISO, most software libraries | US business, broadcast and retail calendars |
Both numbers are shown side by side. If a colleague's "week 33" does not match yours, this is nearly always why — check which convention their system uses before rescheduling anything.
Where week numbers get used
- Manufacturing and logistics — European supply chains schedule almost everything by calendar week ("KW" in German, "sem." in French).
- Project planning — sprint and milestone tracking in Gantt charts and roadmaps.
- Retail and finance — 4-4-5 and 52/53-week fiscal calendars build on week numbers.
- Payroll — weekly and fortnightly pay cycles are usually numbered.
- Date stamping — many components are marked with a two-digit year and week of manufacture.
Writing week dates
The ISO format for a week is 2026-W33, and for a specific day 2026-W33-1, where the final digit is the day of the week with Monday as 1 and Sunday as 7. This is unambiguous and sorts correctly, which makes it a good choice for filenames, report titles and anything a machine will read.
Questions
What makes a week "ISO week 1"?
ISO 8601 defines week 1 as the week containing the first Thursday of January — equivalently, the week containing 4 January. Weeks always run Monday to Sunday.
Why do some years have 53 weeks?
A year has 53 ISO weeks when 1 January is a Thursday, or when it is a Wednesday in a leap year. Everything else has 52, and the tool tells you which case applies.
Why can early January belong to the previous year?
Because a week belongs to whichever year holds its Thursday. 1 January 2027 is a Friday, so it sits in week 53 of 2026 — the ISO week-numbering year, which the tool shows alongside the calendar year.
How does this differ from the US week numbering?
The common US scheme starts weeks on Sunday and calls the week containing 1 January week 1, which frequently differs by one. Both numbers are shown so you can pick the one your organisation uses.