Week Number Calculator
Find the ISO week number, day of year and quarter for any date.
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Pick any date and this calculator returns its ISO 8601 week number together with the day of the year, the calendar quarter, the weekday and how many days are left in the year. It suits project managers, supply-chain and factory planners, and anyone who works with colleagues or systems that schedule by "week 39" rather than by date.
Example: 25 September 2026
- ISO week: Week 39
- Day of year: 268
- Quarter: Q3
- Weekday: Friday
- Days left in year: 97 (365 − 268; the chosen date itself is not counted)
How ISO 8601 numbers the weeks
- Weeks run from Monday to Sunday.
- Week 1 is the week that contains the year's first Thursday. Equivalently, it is the week that contains 4 January.
- Every week belongs entirely to one "ISO year", even if some of its days fall in a different calendar year.
Because of rule 2, a year has either 52 or 53 ISO weeks. A year gets 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks; the previous such years were 2015 and 2020, and the next are 2032 and 2037.
Dates near New Year need care
The calculator shows the week number only, not the ISO year it belongs to, so check the year yourself around January and December:
| Date | Result | Belongs to |
|---|---|---|
| Mon 29 Dec 2025 | Week 1 | ISO year 2026 |
| Thu 1 Jan 2026 | Week 1 | ISO year 2026 |
| Thu 31 Dec 2026 | Week 53 | ISO year 2026 |
| Fri 1 Jan 2027 | Week 53 | ISO year 2026 |
| Sun 3 Jan 2021 | Week 53 | ISO year 2020 |
When writing such dates in the ISO week format, include the ISO year: 1 January 2027 is 2026-W53-5, the fifth day (Friday) of week 53 of 2026.
Other week-numbering systems
Not every tool uses ISO weeks. Excel's WEEKNUM function, by default, starts weeks on Sunday and treats the week containing 1 January as week 1, so its numbers often differ from this calculator by one, and in some years for most of the year. For ISO numbers in Excel, use ISOWEEKNUM or WEEKNUM(date, 21). Many calendar apps also let you choose which day starts the week, which changes their numbering.
Quarters and the Indian financial year
The Quarter row uses calendar quarters: Q1 is January to March, Q2 April to June, Q3 July to September and Q4 October to December. India's financial year runs from April to March, so its quarters are offset: calendar Q2 is FY Q1, Q3 is FY Q2, Q4 is FY Q3, and calendar Q1 is Q4 of the financial year that began the previous April. 25 September 2026 is therefore in Q3 of 2026 but Q2 of FY 2026-27.
Related date tools
Day of year runs from 1 on 1 January to 365, or 366 on 31 December in a leap year. To count the days until a deadline, use the Days Until Calculator; to find the date a set number of days away, use Add or Subtract Days.