How it works
This calculator gives the ISO 8601 week number, the standard used across Europe and in most business and manufacturing calendars. ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4).
Because of that rule, a few days at the very start of January can belong to week 52 or 53 of the previous year, and a few days at the end of December can belong to week 1 of the next year. ISO years have either 52 or 53 weeks.
Formula
Week = ⌊(day of year of that week's Thursday − 1) ÷ 7⌋ + 1
Questions people ask
Why is January 1 sometimes in week 53?
If January 1 falls on a Friday, Saturday or Sunday, that week has most of its days in the previous year, so it counts as the last week of the previous ISO year.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. 2020 and 2026 both have 53 ISO weeks.
Is the US week number different?
It can be. Many US calendars start weeks on Sunday and call the week containing January 1 week 1, which can differ from the ISO number by one.