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Week Number & Start/End Dates - Online ISO 8601

Calculator Reference calculator date ISO 8601 week number
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Current ISO Week
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Date → ISO Week

Select any date to find its ISO 8601 week number and date range.

ISO Week → Date Range

Enter an ISO week number and year to get its date range.

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ISO Week Start (Monday) End (Sunday) Days Status
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Frequently Asked Questions About ISO 8601 Week Numbers
What is the ISO 8601 week numbering system?

ISO 8601 is an international standard for date and time representation. In this system, weeks start on Monday and end on Sunday. The first week of the year (Week 01) is defined as the week containing the first Thursday of the year, which is equivalent to the week containing January 4th. This ensures that every week belongs unambiguously to a single year.

How many weeks are in an ISO year?

Most ISO years have 52 weeks, but some have 53 weeks. A year has 53 ISO weeks if it starts on a Thursday (common year) or on a Wednesday (leap year). For example, 2020 had 53 weeks, while 2021, 2022, 2023, 2024, and 2025 each have 52 weeks. The next 53-week year is 2026.

Can a date belong to a different ISO year than its calendar year?

Yes! This is a common point of confusion. For example, December 31, 2024 (a Tuesday) falls in ISO Week 01 of 2025, because the week containing January 1, 2025 (a Wednesday) includes January 4th and thus becomes Week 1 of 2025. Similarly, January 1st can sometimes belong to Week 52 or 53 of the previous ISO year.

Why do businesses prefer ISO week numbering?

ISO week numbering provides a consistent, unambiguous way to reference weeks across different regions and industries. It's widely used in manufacturing, logistics, payroll, project management, and financial reporting because every week has exactly 7 days and fits cleanly into fiscal planning cycles without partial weeks at year boundaries.

How do I convert between calendar dates and ISO week numbers?

Use this tool! Simply select a date to get its ISO week number and the full date range (Monday–Sunday) of that week. Alternatively, enter a week number and year to find the corresponding date range. The calculation follows the official ISO 8601 algorithm based on the Thursday rule.

What is the difference between US week numbering and ISO 8601?

In the United States, weeks commonly start on Sunday, and Week 1 is simply the week containing January 1st. This leads to inconsistent week lengths at year boundaries (partial weeks). ISO 8601 starts weeks on Monday and uses the Thursday rule, ensuring every week is exactly 7 days and belongs entirely to one year.

What does the format "2025-W01" mean?

This is the standard ISO 8601 week notation. 2025 represents the ISO year, W stands for "Week," and 01 is the two-digit week number (always zero-padded). So "2025-W01" means Week 1 of ISO year 2025, which runs from Monday, December 30, 2024 to Sunday, January 5, 2025.

Which countries use ISO 8601 week numbering?

ISO 8601 is widely adopted across Europe (especially Germany, Netherlands, Scandinavia, and the UK), as well as in Australia, New Zealand, and many parts of Asia. It's the standard for international business communication, airline scheduling, and software development worldwide.

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