What is ISO week numbering (ISO 8601)?

ISO 8601 is an international date/time standard. Under its week numbering system, weeks start on Monday and week 1 is the week that contains the first Thursday of the year. This means January 1 can sometimes fall in week 52 or 53 of the previous year.

How many weeks are in a year?

Most years have 52 ISO weeks. Years where January 1 or December 31 falls on a Thursday (or leap years where January 1 falls on a Wednesday or Thursday) have 53 weeks. The calculator handles 53-week years correctly.

Does the ISO week start on Monday or Sunday?

ISO 8601 weeks always start on Monday. If you use a Sunday-based calendar (common in the United States), your week numbers may differ from the ISO result — especially near the start or end of the year.

Can I find the date range for a specific week number?

Yes. Switch to the reverse lookup mode, enter any year and ISO week number (1–53), and the calculator shows you the full Monday-to-Sunday date range for that week.

Why does my week number differ from another calendar app?

Different systems use different rules. The US calendar often starts weeks on Sunday, and some systems use simple counting from January 1, neither of which matches ISO 8601. This tool strictly follows ISO 8601, which is the standard used across Europe and in most business contexts.

Week Number Calculator – Find ISO Week for Any Date

Find the ISO 8601 week number for any date in seconds — and look up the full date range for any week number to simplify project planning and reporting.

Current Week

Week -- of ----

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What is this tool?

A week number calculator determines which numbered week of the year a given date falls in, using the ISO 8601 standard. Under ISO 8601, weeks always start on Monday, and week 1 is defined as the week containing the year's first Thursday. This standard is widely used in European business, manufacturing, logistics, and academic reporting. The tool also works in reverse — enter a year and week number and it shows you the exact Monday-to-Sunday date range for that week, saving time when building project schedules or payroll periods.

How to use this tool?

1. Click the date picker and select any date, or type it manually in the input field. 2. The ISO week number, the day of the week, and the year are displayed instantly. 3. Use the "Previous week" and "Next week" navigation arrows to step through adjacent weeks. 4. To look up a week by number, switch to the "Find by week" mode, enter the year and week number, and see the Monday–Sunday date range. 5. Use the current-date shortcut button to jump back to today's week instantly. 6. Copy or note the week number for use in project plans, timesheets, or reports.

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Week Number Calculator – Find ISO Week for Any Date

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What is this tool?

How to use this tool?

Frequently Asked Questions

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