ISO week · Monday–Sunday

ISO week number & week calendar

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Year calendar

About ISO weeks

This tool follows the ISO‑8601 definition: weeks start on Monday, and week 1 is the week that contains the year’s first Thursday (equivalently, the week that contains January 4). Some years therefore have 53 ISO weeks.

How we calculate

  1. Convert the given date to a UTC‑based date (avoids DST issues).
  2. Shift to the Thursday of the same week.
  3. Compute the number of weeks since January 1 of that (ISO) year and take the ceiling.
  4. Render a Monday–Sunday grid; each row shows its ISO week number.

This matches the common algorithm used in ISO‑8601 week numbering.

Edge cases

  • Years that start on Thursday or leap years that start on Wednesday have ISO week 53.
  • January 1 can belong to the last ISO week of the previous year if it falls on Friday–Sunday.
  • ISO weeks always start on Monday, regardless of locale.

Examples

  • 2025‑01‑01 (Wed) → ISO week 01 of 2025.
  • 2020‑12‑31 (Thu) → ISO week 53 of 2020.
  • 2016‑01‑01 (Fri) → ISO week 53 of 2015.

FAQ

What is the ISO week number?

The ISO week number identifies the week of the ISO week‑date year (which may differ from the calendar year near New Year). Weeks run Monday–Sunday.

Why do some years have 53 weeks?

If the year starts on Thursday (or it’s a leap year starting on Wednesday), the ISO year contains 53 weeks; otherwise it contains 52.

Is Monday always the first day of the week?

Yes—under ISO‑8601, Monday is day 1, regardless of country‑specific calendars.