How to use the Week Number
- See today’s week number at the top.
- Pick any date to get its week number.
- Browse the table of all weeks in a year.
How ISO week numbers work
Under the ISO 8601 standard, weeks start on Monday and week 1 is the week that contains the first Thursday of the year (equivalently, the week containing 4 January). A year has 52 or 53 weeks. Late December dates can belong to week 1 of the next year, and early January dates to week 52 or 53 of the previous year.
Other systems
The US often counts weeks starting on Sunday with week 1 containing 1 January, which can give a different number. Most of Europe and business software use ISO weeks.
Frequently asked questions
How many weeks are in a year?
52 in most years, 53 in “long years” such as 2020 and 2026.
Does week 1 always contain January 1?
No. In ISO numbering, week 1 is the first week with at least four days in the new year.