Week Number Calculator
Find the ISO week number and ISO week-year for any calendar date.
How to Use the Week Number Calculator
This calculator converts any date into its ISO 8601 week number and ISO week-year. ISO weeks begin on Monday, making the tool useful for international schedules, reporting, manufacturing calendars and project planning.
Example
A date near New Year may belong to the final ISO week of the previous year or the first ISO week of the next year. This is why the tool reports both the week number and ISO week-year.
Factors That Affect the Result
ISO week 1 is the week containing the year’s first Thursday. Because of this rule, the ISO week-year can differ from the ordinary calendar year for dates around late December and early January.
Frequently Asked Questions
Do ISO weeks start on Monday?
Yes.
Can an ISO year have 53 weeks?
Yes. Some years include ISO week 53.
Can the ISO year differ from the calendar year?
Yes, especially around New Year.
Understanding Week Number Calculator Results
This tool is designed for a specific calendar or time-planning task, so the result depends on the exact dates, times and conventions entered. Review boundaries such as midnight, week starts, month lengths and schedule assumptions when they apply. For planning across time zones, daylight-saving transitions or organization-specific calendars, verify those rules separately rather than treating a simple elapsed-time result as universal.
ISO Week-Year Edge Cases
This tool is designed for a specific calendar or time-planning task, so the result depends on the exact dates, times and conventions entered. Review boundaries such as midnight, week starts, month lengths and schedule assumptions when they apply. For time zones, daylight-saving transitions or organization-specific calendars, verify those rules separately rather than treating a simple calendar result as universal.
Calculation Details and Edge Cases
ISO week numbering can place the first days of January in the previous ISO week-year, or the last days of December in the next one. ISO weeks start on Monday and week 1 is the week containing January 4, so the ISO week-year should be shown alongside the week number when dates sit near year boundaries.