Week Number Calculator
Enter any date and find its ISO week number, plus the Monday-to-Sunday dates that week covers.
Monday, September 7 to Sunday, September 13
70% through the year
This tool returns the ISO 8601 week number for any date, along with the start and end of that week, the day of year, and how far through the year you are.
Week numbers come up constantly in payroll, project scheduling, reporting, and sprint planning. The ISO standard is used by most business software and spreadsheet tools, with weeks running Monday to Sunday and the first week of the year defined as the one containing the first Thursday.
How ISO week numbers work
ISO week 1 is the week containing the first Thursday of the year, which means it always includes January 4th. Weeks run Monday to Sunday. Because of this, January 1st can fall in week 52 or 53 of the previous year, and the last days of December can be in week 1 of the next year. The tool shows the ISO year alongside the week number when it differs from the calendar year.
Questions
Why does the year shown differ from the calendar year?
For dates at the very start or end of January and December, the ISO week can belong to a different ISO year than the calendar year. For example, January 1 2016 was in ISO week 53 of 2015. This is correct behaviour, not a bug.
Do weeks always start on Monday?
In ISO 8601, yes. Some countries and organisations use Sunday as the start of the week. If you need Sunday-based week numbers, the ISO figure will typically be one less than what you expect.
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