Week Number Calculator
Find the ISO-8601 week number for any date, and the dates covered by any week number.
Formula
ISO-8601: week 1 contains the first Thursday of the year
Tips
- ISO-8601 weeks start on Monday, and week 1 is the one containing the first Thursday of January.
- Early January can therefore fall in week 52 or 53 of the previous ISO year — which is why the ISO year is shown separately.
- The US convention differs: weeks start on Sunday and week 1 contains 1 January. Spreadsheets often default to that, so check before reconciling.
Frequently asked questions
Why does 1 January sometimes show as week 52?
If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous ISO year under ISO-8601. That is intentional — it keeps every week exactly seven days long.