Week number — ISO 8601 and US conventions
The ISO 8601 week number for any date, alongside the US convention, the day of year, the quarter and the Monday-to-Sunday range that week covers.
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ISO 8601 weeks start on Monday and week 1 is the one containing the first Thursday, which means early January can fall in week 52 or 53 of the previous ISO year. The US convention starts weeks on Sunday and always puts 1 January in week 1 — the two disagree most years.
How to use
- Pick a date. Defaults to today.
- Read both week numbers. ISO 8601 and the US convention differ most years.
- Check the week range. The Monday and Sunday that bracket the ISO week.
Questions
What is the ISO 8601 week number?
Weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of January — equivalently, the week containing 4 January. It is the standard used across Europe and in most business software.
Why can 1 January be in week 52 or 53 of the previous year?
Because ISO weeks never split: a week belongs entirely to the year containing its Thursday. If 1 January is a Friday, Saturday or Sunday, that week belongs to the previous ISO year. 1 January 2021 fell in week 53 of 2020.
How does the US convention differ?
US weeks start on Sunday and week 1 always contains 1 January, however few days that is. So the two systems disagree in most years, sometimes by a full week.
Can a year have 53 weeks?
Yes. An ISO year has 53 weeks when 1 January or 31 December falls on a Thursday. It happens roughly every five to six years.