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Week Number Finder - Online ISO Week of Year

Productivity Reference calendar date ISO week number
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Week Number Finder

Instantly find the ISO week number for any date, or convert a week number to a date range. Fully compliant with ISO 8601.

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Select a date to see its week range
Enter a year and week number to see the corresponding Monday–Sunday range.

Frequently Asked Questions

The ISO week date system is part of ISO 8601. It defines weeks starting on Monday, and week 1 of a year as the week containing the first Thursday of that year. This results in every week being assigned to a single year, even if it spans two calendar years.

Simple week counts (e.g., dividing day of year by 7) create inconsistencies around year boundaries. The ISO system guarantees each week belongs to one year, widely used in business, manufacturing, and financial planning across Europe and globally.

Week 1 is the week that contains January 4th, or equivalently the week with the first Thursday of the year. This ensures that the first days of January are not isolated into a tiny partial week.

Yes. A year has 53 ISO weeks if it starts on a Thursday (common year) or Wednesday (leap year). This happens roughly every 5–6 years. For example, 2020 had 53 weeks.

Because the ISO week year can differ from the calendar year. If December 31 falls in a week where the Thursday lies in the new year, that entire week belongs to week 1 of the next ISO year. For instance, December 31, 2024, is part of ISO week 1 of 2025.

Many European financial systems, supply chain logistics, project management (especially using tools like Jira and Microsoft Project), manufacturing roadmaps, and payroll calendars use ISO week numbers for clarity and consistency.
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