All tools · Date and time tools
ISO-8601 week number for any date.
Free to use, no sign-up required. Week Number is one of 198 tools built into Xenition, alongside a chat that can use them for you.
Gives the ISO 8601 week number for any date, the week numbering used in European calendars, payroll and many planning systems.
ISO weeks run Monday to Sunday, and week 1 is the week containing the year’s first Thursday. The tool moves the date to the Thursday of its own week, then counts which week of that Thursday’s year it falls in. Because of this, a few days at the start of January can belong to week 52 or 53 of the previous year, and late December days can be week 1 of the next year.
27 September 2026 is week 39. 30 December 2024 is week 1 (of 2025), and 1 January 2027 is week 53 (of 2026).
In years where 1 January falls on a Friday, Saturday or Sunday, those days belong to the last ISO week of the previous year. The tool shows the week number only, so remember the year may be the one before.
Many US calendars start weeks on Sunday and count the week containing 1 January as week 1. ISO weeks start on Monday and use the first-Thursday rule, so the two often differ by one.
You do not have to open Week Number to use it. Ask Xenition in plain language and it runs the right tool, then carries the answer into whatever you are working on — a document, a spreadsheet, a deck.