Check what week it is today, or select any date
Under the ISO 8601 standard, the week number is based on three fixed rules: weeks always start on Monday; Week 1 is the week that contains the first Thursday of the year; and a year has either 52 or 53 weeks depending on how the days fall.
In practice: find the Thursday of the week containing your target date, then count how many full weeks have passed since the start of that ISO year. That count is your week number. The calculator above applies this algorithm instantly for any date you select.
Two systems are in common use worldwide, and they sometimes give different results for the same date — which causes real confusion in international teams and software:
| Feature | ISO 8601 (international) | US / Sunday-based |
|---|---|---|
| Week starts on | Monday | Sunday |
| First week rule | Contains first Thursday | Contains January 1st |
| Used in | Europe, Australia, ISO systems | United States, some Latin America |
A concrete example: January 1, 2023 fell on a Sunday. Under the US system that is Week 1 of 2023. Under ISO 8601 that Sunday belongs to Week 52 of 2022, because the week's Monday was December 26, 2022.
Another common surprise: January 1–4, 2026 all fall in Week 53 of 2025, not Week 1 of 2026. That is because the first Thursday of 2026 is January 8 — so Week 1 of 2026 only starts on Monday January 5.
Most years have 52 ISO weeks. A year gets a 53rd week when January 1st falls on a Thursday (or on a Wednesday in a leap year):
| Year | ISO weeks | January 1 falls on |
|---|---|---|
| 2024 | 52 | Monday |
| 2025 | 52 | Wednesday |
| 2026 | 53 | Thursday |
| 2027 | 52 | Friday |
| 2032 | 53 | Thursday |
Business and finance: Many companies structure quarterly reports and sprint planning around week numbers. Weeks 1–13 map to Q1, Weeks 14–26 to Q2.
Construction and logistics: Large projects use week numbers in Gantt charts. A delivery scheduled for W32 is unambiguous regardless of month.
Academic calendars: Universities across Germany, the Netherlands, and Scandinavia publish semester schedules in week numbers. KW15 = ISO Week 15.
The calculator above detects today's date automatically and shows the current ISO week number without any input needed.
If January 1st falls on a Friday, Saturday, or Sunday, those days cannot be Week 1 under ISO rules — they belong to the final week of the previous year. January 1–4, 2026 all fall in Week 53 of 2025.
2026 has 53 ISO weeks, because January 1 falls on a Thursday — the trigger condition for a 53-week year.
ISO weeks start on Monday; US weeks start on Sunday. ISO Week 1 contains the first Thursday of the year; US Week 1 contains January 1st. They can differ by one week in early January.
No. ISO 8601 is standard across Europe and internationally. The US and Canada use a Sunday-based system. Always specify which standard you are using when sharing week numbers across borders.