How to use Week Number Calculator
The week number calculator displays the ISO 8601 week number for any date, the start and end of that week, the day of the year (day 1–366), the quarter, the days remaining in the year and whether the date falls in a leap year. ISO week 1 is defined as the week containing the first Thursday of the year.
- Select a date or use today's date.
- View the week number, week boundaries, day of year, quarter and year summary.
- Switch dates near New Year to check whether the ISO week belongs to the previous or next ISO year.
- Use the week start and end dates to label sprint plans, timesheets, and payroll periods consistently.
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ISO 8601 week numbering explained
The ISO 8601 standard defines weeks starting on Monday. Week 1 is the week containing January 4th (equivalently, the week with the year's first Thursday). This means December 29–31 may belong to week 1 of the following year, and January 1–3 may belong to week 52 or 53 of the previous year. The ISO year can differ from the Gregorian calendar year at year boundaries.
Worked examples
Independence Day 2025
Inputs: July 4, 2025 (a Friday)
Result: ISO week 27
First week edge case
Inputs: January 1, 2025
Result: ISO week 1 (belongs to the week starting Dec 30, 2024)
Week-numbering systems compared
| System | Week starts on | Week 1 rule |
|---|---|---|
| ISO 8601 | Monday | Week containing the year's first Thursday (and Jan 4) |
| US / North America | Sunday | Week containing January 1 |
| Middle East | Saturday | Week containing January 1 |
Glossary
- ISO 8601
- International standard for date and time representations, including week numbering.
- Leap year
- A year with 366 days (February has 29 days) occurring every 4 years, with century exceptions.
- ISO week year
- The year associated with an ISO week number, which can differ from the calendar year around New Year.
- Week boundary
- The exact Monday start and Sunday end dates for a specific week number.
Related reading
Frequently Asked Questions
Why use Week Number Calculator?
- Gives the ISO 8601 week number for any date, resolving the year-boundary ambiguity correctly
- Displays the week's Monday start and Sunday end so sprint or payroll periods are immediately clear
- Identifies the fiscal quarter and day-of-year alongside the week number in a single lookup
- Flags leap years and remaining days in the year for annual planning and reporting
Common use cases
- Identify which sprint number a release falls in when your team uses ISO week-based sprint labels
- Complete a weekly timesheet that requires an ISO week number in the header
- Check whether a late-December date belongs to the current or following year's week 1 for payroll
- Confirm which quarter a transaction falls in for quarterly financial reporting
- Map ISO week numbers to project milestones when building an annual delivery calendar
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