Enter a date to get its ISO 8601 week number — the “week 34” style used across European business, logistics and planning — along with the Monday-to-Sunday dates of that week and how many weeks the year contains. It all runs on your device.
Pick a date to see its ISO week number.
ISO 8601 weeks start on Monday; week 1 contains the year’s first Thursday.
How it works
ISO 8601 defines weeks starting on Monday, with week 1 being the week that contains the year’s first Thursday (equivalently, the week holding 4 January). The tool finds the Thursday of your date’s week to pin down the number, so the result matches calendars, spreadsheets and shipping systems that use ISO weeks.
Because of that rule, the week-year can differ from the calendar year at the edges: 31 December can belong to week 1 of the next year, and 1 January can belong to week 52 or 53 of the previous one. The tool always shows the week-year next to the number, and tells you whether the year has 52 or 53 weeks.
Practical examples
Which week is a date in?
Enter 2024-03-14 and you get week 11 of 2024, running Monday 2024-03-11 to Sunday 2024-03-17 — ready to drop into a week-numbered plan.
A year-end date that rolls forward
2024-12-30 is a Monday that already belongs to week 1 of 2025 under ISO rules. The week-year label makes that explicit so you don’t file it under 2024 by mistake.
Does this year have 53 weeks?
Check any date in 2020 and the tool shows the year has 53 ISO weeks — useful for annual planning and payroll that runs on week numbers.
Frequently asked questions
Which week-numbering standard does this use?
ISO 8601, the international standard used across Europe. Weeks start on Monday and week 1 is the one containing the first Thursday of the year. It’s the numbering behind “KW” in German, “semaine” in French and week numbers in most European calendars.
Why can 1 January be in week 52 or 53?
Because ISO week 1 must contain the year’s first Thursday. If 1 January is a Friday, Saturday or Sunday, it falls in the last week of the previous year. The tool shows the correct week-year alongside the number to avoid confusion.
Why do some years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it’s a leap year starting on a Wednesday. Most years have 52. The tool tells you which, so long-range plans account for the extra week.
Is this the same as the US week numbering?
No. The common US convention starts weeks on Sunday and counts 1 January as week 1, which often differs from ISO by a week. This tool uses ISO, the standard in Serbia and Europe; a US-style option isn’t included.
What dates does the Monday–Sunday range show?
The Monday that begins your date’s ISO week and the Sunday that ends it. Any date you enter maps to the same seven-day block, which is handy for booking or reporting against a whole week.
What’s the week number today?
Enter today’s date to see it. The tool doesn’t assume the current date so results stay stable and shareable, but picking today gives the current ISO week immediately.
Does the range depend on my time zone?
No. The week number and its Monday–Sunday range are computed from the calendar date you enter, independent of time zone, so everyone sees the same result for the same date.
Is the date I enter kept private?
Yes. Everything is computed in your browser; nothing is uploaded and analytics never receives the date.
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