ISO Week Number Calculator
Find the ISO 8601 week number, ISO year, weekday, day of the year and quarter for any date, and the Monday to Sunday dates of any ISO week.
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Input
ISO week number and day of the year
Output
2026-W44-2
| ISO week | 2026-W44-2 |
|---|---|
| Weekday | Tuesday |
| Day of year | 300 of 365 |
| Quarter | Q4 |
| Weeks in ISO year 2026 | 53 |
| Unix time (midnight UTC) | 1793059200 |
2026-W01
2025-12-29 to 2026-01-04
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday, so 1 January can belong to week 52 or 53 of the previous ISO year. Use the ISO year (PHP's date('o'), not 'Y'; strftime's %G, not %Y) next to the ISO week, or late-December dates get labelled with the wrong year. MySQL's WEEK(date, 3) and PostgreSQL's EXTRACT(WEEK) follow ISO.
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About this tool
Method, assumptions, practical next steps, and limits.
How ISO weeks work
ISO 8601 weeks start on Monday, and week 1 is the week that contains the year's first Thursday, equivalently the week with 4 January. So 1 January can fall in week 52 or 53 of the previous year, and 31 December can be in week 1 of the next. A year has 52 or 53 ISO weeks.
The year bug
Combining an ISO week with the calendar year mislabels dates around New Year: 2024-12-30 is week 1 of ISO year 2025. Use the ISO year next to the ISO week: o rather than Y in PHP's date(), %G rather than %Y in strftime. MySQL's WEEK(date, 3) and PostgreSQL's EXTRACT(WEEK) follow ISO.