ISO week numbers make “week 1” portable across teams, reports, and systems. ISO weeks begin on Monday, and week 1 is the week containing January 4—the same week that contains the year's first Thursday.
Why the ISO year matters
Near New Year, an ISO week can belong to a different year than its calendar date. For example, 2027-01-01 belongs to ISO week 53 of 2026. When a report or filename uses a week number, keep the ISO year with it: 2026-W53 is unambiguous; “week 53” is not.
- ISO weekdays run from 1 for Monday to 7 for Sunday.
- Most ISO years have 52 weeks; some have 53.
- The tool shows the ISO year, week number, weekday, day of year, and calendar quarter together.
Use calendar dates for ordinary monthly reporting, ISO weeks for week-based operations, and an explicit fiscal calendar when the business year does not begin in January.
Frequent questions:
- Q: What week is it right now?
- A: Open /week-number — the date input is pre-filled to today, so the headline shows 'Week N of YYYY' for the current ISO week. The same panel also shows the weekday, the day-of-year, the quarter, and how many weeks remain in the ISO year.
- Q: What is the difference between ISO year and calendar year?
- A: ISO week 1 of a year is the week containing that year's first Thursday. So the last few days of December may sit in ISO week 1 of the next year (e.g. 2025-12-29 is ISO 2026-W01-1), and the first few days of January may sit in the last ISO week of the previous year. The tool always prints both.
- Q: How many weeks are in 2026?
- A: 53 — because January 1 2026 falls on a Thursday. Most ISO years have 52 weeks; the 53-week years are those whose January 1 is Thursday, or leap years whose January 1 is Wednesday.