ISO Week & Quarter
runs in your browserWhat week is it? The ISO week number of any date, the dates of any week or quarter, fiscal quarters, and every week of the year with its Monday and Sunday.
Type a date, an ISO week, a quarter or a day of the year. Empty, it shows today.
For example 2026-10-14, 2026-W42, 2026-W42-3, 2026-Q4 or 2026-287.
about this tool
How an ISO week is counted
ISO 8601 numbers the weeks of a year from Monday to Sunday. Week 1 is the week that holds the year's first Thursday, and so always holds 4 January. That rule keeps every week whole: a week is never split between two years. The price is at the edges. 1 January 2027 is a Friday, so it falls in week 53 of 2026, and 29 December 2025, a Monday, is the first day of week 1 of 2026.
A date written as an ISO week date, 2026-W42-3, gives the week-numbering
year, the week and the day, Monday being 1. Type one, or a plain date, a
week (2026-W42), a quarter (2026-Q4) or a day of the year (2026-287),
and the tool shows the rest: the week's Monday and Sunday, the quarter and
its dates, the day of the year, and how many days are left.
Quarters and fiscal years
Calendar quarters run January to March, April to June, July to September and October to December. A fiscal year can start in any month; choose April, July or October and each date also shows its fiscal quarter, with the fiscal year written as the two years it spans, since some organisations name it by its first year and some by its last.
The US week number
Many US calendars, and Excel's WEEKNUM by default, count weeks from
Sunday, with week 1 being the week that holds 1 January, however few of
its days are in January. That number is shown as well, because the two are
easily confused.
In a spreadsheet or code
| Where | ISO week |
|---|---|
| Excel, Google Sheets | =ISOWEEKNUM(A1) |
| PostgreSQL | to_char(d, 'IYYY-"W"IW-ID') |
| MySQL | DATE_FORMAT(d, '%x-W%v') |
| Python | d.isocalendar() |
| JavaScript (date-fns) | getISOWeek(d), getISOWeekYear(d) |
The week-numbering year matters as much as the week: pair IW with IYYY
in PostgreSQL, %v with %x in MySQL, and never with the calendar year,
or the last days of December come out as week 1 of the wrong year.
Every week of the year is listed below the result, Monday to Sunday, and downloads as CSV for a planner. For the days between two dates, use the date difference calculator; for a calendar with week numbers to print, the printable calendar.
questions
- How are ISO week numbers counted?
- By ISO 8601: weeks start on Monday, and week 1 of a year is the week holding its first Thursday, which is also the week holding 4 January. So the first days of January can belong to the last week of the year before, and the last days of December to week 1 of the next.
- Why do some years have 53 weeks?
- A year has 52 weeks and one or two days over, and the days add up. A year that starts on a Thursday, or a leap year that starts on a Wednesday, has 53 ISO weeks: 2020, 2026 and 2032 are such years.
- Why is my calendar’s week number different?
- Many calendars in the United States count weeks from Sunday with week 1 holding 1 January, as Excel’s WEEKNUM does by default. The two systems often differ by one, and around the new year by more. The tool shows both; ISO weeks are the ones used in Europe and in business planning.
- Which fiscal year does a quarter belong to?
- Choose the month your fiscal year starts: April in India, the United Kingdom and Japan, July in Australia, October for the US federal government. The tool gives the fiscal year as the two calendar years it spans, FY 2026–27, since organisations name it by either its first or its last year.
- Does it depend on my time zone?
- Only for what today is, which comes from your computer’s clock. Every other date is calendar arithmetic with no time of day, so no time zone or daylight-saving change can shift it.