ISO Week Date Converter
Convert dates to and from ISO 8601 week numbers.
How to use
- 1 To get a week number, pick a calendar date on the left.
- 2 Read the ISO week date in YYYY-Www-D form, plus the plain-English week and weekday.
- 3 To reverse it, enter a week-year, week number and weekday on the right.
- 4 Copy whichever result you need with one click.
About ISO Week Date Converter
The ISO Week Date Converter translates between ordinary calendar dates and the ISO 8601 week-date format used throughout business, manufacturing and software.
Give it a date like 2026-06-24 and it returns 2026-W26-3 — week 26, weekday 3 (Wednesday) — and going the other way, enter a week-year, week number and weekday to get the exact calendar date back.
ISO weeks always start on Monday, and week 1 is defined as the week containing the year's first Thursday.
That rule produces results that surprise people at year boundaries: the 1st of January can belong to the final week of the previous year, and the 31st of December can belong to week 1 of the next.
This tool gets those edge cases right by anchoring every calculation on the Thursday of the week, so the week-numbering year it reports is the genuine ISO year, not just the calendar year.
It also validates strictly: impossible dates such as 30 February are refused, and week 53 is rejected for any year that only has 52 ISO weeks, so you never get a silently wrong answer.
All calculations run in UTC inside your browser with no uploads, making the tool fast, private and fully usable offline.
FAQ
Why can 1 January fall in week 52 or 53 of the previous year?
ISO week 1 is the week containing the year's first Thursday. If 1 January is a Friday, Saturday or Sunday it belongs to the last week of the prior ISO year.
How many weeks can an ISO year have?
Either 52 or 53. Years starting on a Thursday (or a leap year starting Wednesday) have 53 weeks; the tool rejects week 53 for 52-week years.
What does a weekday number mean here?
ISO weekdays run 1 to 7, where 1 is Monday and 7 is Sunday, unlike the common Sunday-first convention.