Week Number Calculator
Find the ISO week number (1–53) for any date.
How to use this tool
- Enter date in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your iso week number and the full breakdown beneath it.
Find the ISO week number (1–53) for any date.
How it works
This calculator tells you the ISO week number — a standardised week count from 1 to 53 — for any calendar date you enter. ISO 8601 defines week 1 as the week containing the first Thursday of the year, which means weeks always start on Monday and the first or last few days of January can belong to the previous or following year's week count.
Enter any date and the tool returns the ISO week number, the year that week belongs to (which may differ from the calendar year for dates in early January or late December), and the Monday and Sunday dates that bound that week.
Week numbers are widely used in European business planning, logistics, manufacturing schedules, payroll systems, and project management. If a supplier quotes "delivery in W38" you can use this tool to find the exact calendar dates for that week.
Note that the US and some other countries use a different week-numbering convention where weeks start on Sunday and week 1 is the week containing January 1st. This tool follows the international ISO 8601 standard, which is the most common in business contexts.
Worked example
Find the ISO week for a project deadline
- Enter the deadline date, e.g. 2025-09-15.
- The calculator returns: ISO Week 38, year 2025.
- It also shows the week runs from Monday 15 Sep to Sunday 21 Sep 2025.
The deadline falls in W38 2025, running 15-21 September — useful for scheduling sprints.
Common mistakes to avoid
- Assuming the ISO week year always matches the calendar year — it can differ by one for dates in late December or early January.
- Using a Sunday-based US week convention when collaborating with European partners who use ISO Monday-based weeks.
- Expecting all years to have exactly 52 weeks — some years have 53 ISO weeks.
Key terms
- ISO 8601
- The international standard for representing dates and times. Its week numbering system defines Monday as the first day of the week and week 1 as the week containing the first Thursday of the year.
- ISO week year
- The year a given ISO week belongs to. For dates in early January or late December, the ISO week year can differ by one from the Gregorian calendar year.
- Week 53
- Some years have 53 ISO weeks instead of 52. This occurs when 1 January falls on a Thursday (or on a Wednesday in a leap year).
Frequently asked questions
- Which standard does this use?
- ISO 8601: week 1 is the week containing the first Thursday of the year. Weeks start on Monday.