ISO week numbers explained
Much of Europe and Asia plans in week numbers: delivery in week 12, holidays in weeks 31 and 32, the sprint that ends in week 44. Those numbers follow ISO 8601, the international standard, in which weeks start on Monday and week 1 is the week with the year's first Thursday.
Pick any date and this tool shows its ISO week number, the ISO week-numbering year it belongs to, the weekday and the day of the year.
How to use it
- Pick a date, or keep today's date.
- Read the ISO week and the ISO year.
- Check the ISO year around New Year: early January can belong to the last week of the previous year, and late December to week 1 of the next.
- Use the day of the year when you need an ordinal date such as 2026-299.
Example: a January date that belongs to the year before
2021-01-03
ISO week 53 of 2020, Sunday, day 3
3 January 2021 was a Sunday, the last day of the week that began on Monday 28 December 2020. That week's Thursday was 31 December, so the whole week counts as week 53 of 2020.
The rule for week 1
ISO week 1 is the week that contains the first Thursday of January. Equivalently, it is the week that contains 4 January, or the first week with at least four days in the new year. As a result, 1 to 3 January can belong to the last week of the previous year, and 29 to 31 December can belong to week 1 of the next.
A year has 52 or 53 ISO weeks. It has 53 when it starts on a Thursday, or on a Wednesday in a leap year, as 2020 and 2026 do.
Writing week dates
The standard notation is 2026-W44 for a week and 2026-W44-1 for its Monday. Always pair the week with the ISO year rather than the calendar year, because they differ around New Year; this is a classic bug in date code that formats the year with the wrong pattern, such as YYYY instead of yyyy in Java.
Other week systems
The United States and some other countries usually start weeks on Sunday and count week 1 as the week containing 1 January, so their week numbers can differ by one from ISO. Spreadsheets offer both: in Excel, ISOWEEKNUM gives the ISO number while WEEKNUM defaults to the US system.
Questions
Does 2026 have a week 53?
Yes. 2026 starts on a Thursday, so it has 53 ISO weeks, and week 53 runs from 28 December 2026 to 3 January 2027.
Which day does an ISO week start on?
Monday. Sunday is day 7, the last day of the week.
Is anything uploaded?
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