How is the ISO week number defined?

Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. This means the first few days of January can fall in the last week of the previous year, and late December can fall in week 1 of the next year.

Why does the ISO year sometimes differ from the calendar year?

Because a week belongs to the year that contains its Thursday. For example, 1 January 2027 is a Friday, so it belongs to ISO week 53 of 2026, and its ISO year is 2026 even though the calendar year is 2027.

Is my date sent anywhere?

No. The calculation runs entirely in your browser and nothing is uploaded.

ISO Week Number Calculator

Find the ISO 8601 week number for any date.

ISO 8601 week number for any date

Enter a date and get its ISO 8601 week number in the standard year-Wnn format, along with the weekday and the Monday-to-Sunday span of that week.

ISO weeks always start on Monday, and week 1 is the week containing the year’s first Thursday. That rule is why a date in early January can belong to the previous year’s final week, and why some years have 53 weeks. Everything is computed locally in your browser.