Pick a date to see its week number. Most of the world, including New Zealand, Australia, the UK and Europe, uses ISO weeks, which start on Monday. The US usually counts weeks from Sunday, with week 1 being the week that contains 1 January. Both are shown.
| Week | Monday | Sunday |
|---|
ISO 8601 weeks start on Monday. Week 1 is the week containing the year’s first Thursday, which is the same as the week containing 4 January. So 1–3 January can belong to the last week of the previous year, and 29–31 December can fall in week 1 of the next. A year has 52 ISO weeks, or 53 when it starts on a Thursday (or on a Wednesday in a leap year).
US-style weeks start on Sunday, and week 1 is simply the week that contains 1 January, even if it’s only one day long. That’s why the two numbers can differ by one.
The day of the year counts 1 January as day 1, so 31 December is day 365, or day 366 in a leap year.
Tap “Today” and the ISO week and the US week are both shown. In New Zealand, Australia, the UK and Europe, the ISO number is the one used on calendars and in business.
Under ISO rules a year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year. 2026 is one: it has a week 53 running from Monday 28 December 2026 to Sunday 3 January 2027.
Most of Europe, the UK, New Zealand, Australia and much of Asia use ISO 8601 (Monday start). The US, Canada and some other countries more often count from Sunday with week 1 containing 1 January.
Yes. Under ISO rules, if 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year. The ISO week-numbering year is shown next to the week number when this happens.
Scroll the week list below the results: it shows the Monday and Sunday of every ISO week in the selected year, with the chosen week highlighted.
