Week Number Calculator
Find the ISO week number for any date.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Week Number Calculator
Select any date and the calculator returns its ISO 8601 week number, the ISO year it belongs to, and the day of the week.
- Pick a date. The calculator defaults to today.
- Read the week number. ISO weeks are numbered 1 through 52 (or 53 in some years).
- Note the ISO year. For dates near the end or beginning of a year, the ISO year may differ from the calendar year. For example, January 1, 2016 falls in ISO week 53 of 2015.
About ISO 8601 Week Numbers
ISO 8601 is the international standard for representing dates and times. Under this standard, weeks begin on Monday and the first week of the year is the week containing the year's first Thursday. This means week 1 always contains January 4.
As a result, a few days at the end of December may belong to week 1 of the following year, and a few days at the start of January may belong to week 52 or 53 of the previous year. The ISO year shown in the breakdown reflects this, and may differ from the calendar year for those edge-case dates.
Frequently Asked Questions
What is ISO week 1 of the year?
ISO week 1 is the week that contains the year's first Thursday. This always includes January 4. Week 1 can begin as early as December 29 of the previous year or as late as January 4 of the current year, depending on the day of the week January 1 falls on.
Can a year have 53 ISO weeks?
Yes. A year has 53 ISO weeks when January 1 is a Thursday, or when it is a Thursday or Wednesday in a leap year. This occurs about every 5 to 6 years. The years 2015, 2020, and 2026 all have 53 weeks.
Why does the ISO year sometimes differ from the calendar year?
Because ISO weeks start on Monday and Week 1 is defined by where Thursday falls, a few days at the start of January can belong to the last week of the previous ISO year, and a few days at the end of December can belong to Week 1 of the following ISO year. For example, December 31, 2018 was in ISO week 1 of 2019.
Do all countries use the same week numbering system?
No. The ISO 8601 system with weeks starting on Monday is standard in most of Europe and many other countries. The United States typically uses a system where weeks start on Sunday and Week 1 is the week containing January 1, which can produce different week numbers for the same date. This calculator uses the ISO 8601 standard.