ShipScroll

ISO week number calculator

Pick a date to see its ISO week number and that week’s dates, or type a week like 2026-W46 to see when it starts and ends. It also tells you whether the year has 52 or 53 weeks.

It’s for makers planning a launch week, reading week-based reports, or working with teams that schedule by week number. ShipScroll’s launch weeks are ISO weeks, Monday to Sunday.

Free with a ShipScroll account.

How to use the ISO Week Calculator

  1. Pick a date, or type a week such as 2026-W46.
  2. Press Find the week.
  3. Read the week number and its Monday-to-Sunday dates.
  4. Change either field and the answer updates.

How ISO weeks work

  • Weeks start on Monday and end on Sunday.
  • Week 1 is the week with the year’s first Thursday in it, which is always the week containing 4 January.
  • So the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next.
  • A year has 52 weeks, or 53 when it starts on a Thursday (or a Wednesday in a leap year).

Week numbers aren’t the same everywhere

The US and some other countries number weeks differently, starting on Sunday and counting the week of 1 January as week 1. Calendar apps may use either system. ISO 8601 is the international standard, and it’s what this tool and ShipScroll use.

Questions

Why is 1 January in week 52 or 53?

Because ISO week 1 is the week with the year’s first Thursday. If 1 January falls on a Friday, Saturday or Sunday, that week has more days in the old year, so it belongs to the old year’s last week.

How do I write an ISO week?

As the year, a hyphen, W and the two-digit week: 2026-W46. The year is the ISO week-year, which can differ from the calendar year in the first and last days of the year.

Which time zone does it use?

The date you pick is treated as a calendar date, so the time zone doesn’t change the answer.

How do ShipScroll launch weeks line up?

Each launch week is an ISO week, starting at 00:00 UTC on Monday. A product booked for 2026-W46 goes live on that Monday.

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Sources