What week number is it?

The leaf at the top shows today’s ISO week number, worked out from your device’s date. On October 24, 2026 it was week 43 of 2026.

Under ISO 8601 a week runs Monday to Sunday, and week 1 is the week that contains the year’s first Thursday. Equivalently, week 1 is the week with January 4 in it. Most of Europe and much of Asia use this system, and it is what spreadsheets call ISOWEEKNUM.

If January 1 is a Friday, Saturday or Sunday, that week has most of its days in the old year, so it belongs to the old year’s last week. For example, January 1, 2027 is a Friday and falls in week 53 of 2026.

A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. That happens about 71 times every 400 years. 2026 and 2032 have 53 weeks; 2027 has 52.

It can be. A common US convention starts weeks on Sunday and calls the week containing January 1 week 1, so its numbers can be one higher than ISO numbers for part of the year. Excel’s WEEKNUM function uses that system by default, while ISOWEEKNUM uses the ISO rule shown here.