Tool
Week Number Calculator — ISO Week
Pick a date — get the ISO 8601 week number, the year that week belongs to, and the day-of-year count.
What is an ISO week?
ISO 8601 defines week 1 of a year as the week containing the first Thursday. Weeks start on Monday and end on Sunday. A handful of days in late December or early January can belong to a different ISO year than the calendar year — this tool handles that correctly.
How week numbers are used
- European countries widely use week numbers in business and logistics ("delivery in week 32").
- Sprint planning and OKR cadences often reference week numbers.
- School and university timetables in Germany, the Nordics, and the UK label weeks by their ISO number.
Related
Frequently asked questions
Is the week starting Monday or Sunday?
ISO 8601 weeks start on Monday. This tool follows the ISO standard. In some US contexts weeks are counted starting Sunday — those numbers can differ by one.
Why is January 1st sometimes in week 52 or 53?
If January 1st falls on a Friday, Saturday, or Sunday, it belongs to the last ISO week of the previous year. The tool returns the correct ISO year and week pair.
How many weeks does a year have?
Most years have 52 ISO weeks. Some have 53 — including 2026 and 2032.