Week Number Calculator
About Week Number Calculator
What Is a Week Number Calculator?
A week number calculator determines the ISO week number for any given date. ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday and are numbered 1 through 52 or 53. Week numbering is widely used in Europe, business planning, manufacturing schedules, project management, and accounting systems where referring to weeks by number simplifies communication and scheduling.
ISO Week Numbering Rules
The ISO standard defines that week 1 is the week with the year's first Thursday. This means January 1 might belong to the previous year's last week. Similarly, December 28-31 might belong to week 1 of the next year. Most years have 52 weeks, but some have 53 weeks (called long years). The calculator handles all these edge cases automatically.
How to Use This Calculator
Enter a date and click Calculate. The result shows the ISO week number, the ISO year it belongs to, the first and last day of that week, and a mini calendar showing surrounding weeks for context.
Frequently Asked Questions
Why does January 1 sometimes show as week 52 or 53?
Under ISO 8601, week 1 must contain the first Thursday of the year. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year.
Do all countries use ISO week numbering?
ISO week numbering is standard in most of Europe and many international businesses. The US and some other countries often use a different system where week 1 is the week containing January 1 and weeks start on Sunday.
How many weeks are in a year?
Most years have 52 ISO weeks. Years where January 1 is a Thursday, or December 31 is a Thursday, have 53 weeks.
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