"What week is it?" is one of the most-asked calendar questions — especially in Europe, logistics, and business planning, where schedules are built around week numbers instead of dates. This guide explains how week numbers work and how to find the current one in seconds.
For the live answer right now, our today page shows the current ISO week number, and the week numbers reference lists every week of the year with its dates.
How to find the current week number
The fastest way: check our today page — it shows today's ISO week number along with the date, day of year, and more, updated automatically.
To work it out manually you need the ISO 8601 rule, which is the international standard used across Europe and in most business software.
The ISO 8601 week rule
Three simple rules define ISO week numbers:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January.
- A year has either 52 or 53 weeks.
Because of rule 2, the first few days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next year. That surprises a lot of people, but it keeps every week whole.
Why Monday and not Sunday?
ISO 8601 defines Monday as the first day of the week — the convention in most of the world and in business. However, the United States, Canada and Japan often use a different system where the week starts on Sunday and week 1 is simply the week containing 1 January. This means the US "week number" can differ from the ISO week number by one. Our week numbers tool uses the ISO standard.
How many weeks are in a year?
Most years have 52 weeks. A year has 53 ISO weeks only when it starts on a Thursday, or when a leap year starts on a Wednesday. For example:
| Year | ISO weeks |
|---|---|
| 2025 | 52 |
| 2026 | 53 (starts Thursday) |
| 2027 | 52 |
| 2028 | 52 |
See the full breakdown in our how many weeks in 2026 guide.
Where week numbers are used
- Business & manufacturing: production and delivery schedules ("ships in week 38").
- Europe: calendars, payslips and planners routinely print week numbers.
- Project management: sprints and milestones tracked by week.
- Retail & fiscal calendars: the 52/53-week financial year.
Quick answers
What week number is it today? Check the today page — it shows the current ISO 8601 week number, updated automatically.
How is week 1 decided? Under ISO 8601, week 1 is the week containing the first Thursday of the year (the week with 4 January in it).
Do weeks start on Monday or Sunday? ISO 8601 (used in Europe and business) starts weeks on Monday. The US, Canada and Japan often start on Sunday.
Can a year have 53 weeks? Yes. A year has 53 ISO weeks when it begins on a Thursday, or when a leap year begins on a Wednesday.