Enter a date to find its ISO 8601 week number.
Calculated locally in your browser — nothing is sent to our server.
ISO week
Week runs from
How This Tool Works
Uses the international ISO 8601 definition: weeks start on Monday, and week 1 of a year is the week containing that year’s first Thursday. This is NOT the US convention (weeks starting Sunday) — a date very early or late in the year can belong to the adjacent year’s week numbering under ISO rules, and this tool handles that correctly rather than just dividing day-of-year by 7.