Week Number of the Year Calculator Online

This online calculator tells you which week number of the year a date belongs to. It follows the ISO 8601 standard: weeks start on Monday, and Week 1 is the week containing January 4. This ensures consistent, internationally recognized week numbering across different systems and countries.

Week of the Year Calculator (ISO 8601)

Select a date or use today; the tool shows which week of the year it is.

Date

Result

ISO week rule: weeks start on Monday; Week 1 contains January 4 (first Thursday rule).

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How to use

Today’s date is automatically selected, and the result shows the current week of the year.
To check another date, pick it from the calendar field.
Click Calculate to display the ISO week number.
Click Copy to copy the result to your clipboard.
Click Clear to reset the tool and reload today’s date.

Definition of ISO week

ISO 8601 week date — an international standard for calendar weeks. A week always begins on Monday and ends on Sunday. Week 1 is the first week of the year that contains January 4 (or, equivalently, the first Thursday of the year). This prevents short “Week 0” at the start of the year and ensures consistent numbering across borders.

Examples

January 1, 2024 → Week 52 of 2023 (because Jan 1, 2024 is in the last week of the previous ISO year)
January 4, 2024 → Week 1 of 2024
February 15, 2025 → Week 7 of 2025
June 1, 2025 → Week 22 of 2025
December 31, 2025 → Week 1 of 2026 (belongs to next ISO year)

Week Number of the Year Calculator Online

A year can have 52 or 53 weeks

Clarifications

ISO weeks do not always align with calendar years. Early January or late December dates may belong to the previous or next ISO year.
Each ISO year has either 52 or 53 weeks, depending on how the days align.
The standard is used worldwide in scheduling, finance, project management, and software systems.

FAQ

Q: Why does the ISO week sometimes show the previous or next year?
A: Because ISO weeks are aligned by the “first Thursday” rule, a few days at the start or end of a calendar year may belong to the neighboring ISO year.

Q: How many weeks can a year have?
A: Normally 52 weeks, but years that begin on Thursday (or are leap years beginning on Wednesday) have 53 weeks.

Q: Do all countries use ISO weeks?
A: Not always. The US, for example, often counts weeks starting on Sunday and doesn’t use ISO numbering. But ISO 8601 is widely adopted in international standards, the EU, and computing systems.

Q: Is Week 1 always the first week of January?
A: No. If January 1 falls on a Friday, Saturday, or Sunday, those days are considered part of the last ISO week of the previous year. Week 1 starts on the first Monday that includes January 4.

Q: What practical uses does this have?
A: Common uses include payroll cycles, school timetables, project planning, statistical reporting, and international logistics.

Q: Why Monday as the first day of the week?
A: ISO 8601 defines Monday as the first day for consistency. In many European countries, this matches cultural practice, though in some places (e.g., the US) Sunday is often treated as the week start.

Q: Is this tool accurate for leap years?
A: Yes. The ISO rules account for leap years automatically, so calculations remain correct.

Q: What’s the difference between ISO week and simple “week of the year” counting?
A: Non-ISO week numbering often just divides days of the year by 7, starting from January 1. This creates irregular or partial weeks. ISO week numbering avoids this by enforcing a consistent Monday-to-Sunday structure and the Jan 4 rule.

More details

The ISO 8601 system is defined by the International Organization for Standardization. Official documentation is available from ISO.org. It ensures compatibility between countries, computer systems, and industries, which makes it a reliable standard for week numbering worldwide.