ISO 8601 starts the week on Monday; the first week of a year is the one containing its first Thursday.
Numero de Semana ISO 8601 Actual
ISO 8601 puts Monday at the start of the week and defines week 1 as the week containing the first Thursday of the year — which means a few days at the start of January may belong to week 52 or 53 of the previous year. This tool matches the week numbering of many European systems.
The week number is computed locally and refreshes each minute (auto-updates across week boundaries).
Features
- ISO 8601 compliant — Monday-start, Thursday-rule week numbering, consistent with European enterprise systems.
- Week-year shown — Also shows the ISO week-year the week belongs to (which may differ from the calendar year).
- Live refresh — Updates automatically across week boundaries.
How to use
- Open the page to see the current ISO week number and week-year.
FAQ
Why can January 1 show week 52?
By the Thursday rule, if Jan 1 is Fri/Sat/Sun, most of its week falls in the previous year, so it is counted as week 52 or 53 of that year — which is why the ISO week-year can differ from the calendar year.
How is the ISO week number different from the common one?
Many US-style systems start the week on Sunday and count the week containing Jan 1 as week 1; ISO starts on Monday and uses the Thursday rule, so the two can differ by a week at year start/end.