Week Number Calculator

Find the ISO week number for any date, or get the exact date range for any week of the year. Supports ISO-8601 weeks (Monday as the first day of the week).

Defaults to today. Uses ISO-8601 week numbering.

Week information

Week –

ISO week year: –
Calendar year of date: –

Week date range (Mon–Sun)

Start of week (Monday):
–

End of week (Sunday):
–

Quick navigation

How the week number calculator works

This tool uses the ISO-8601 week date system, the same standard used in most business calendars, project plans, and software systems. It works in both directions:

  • Date → Week number: pick any date to see its ISO week number and the Monday–Sunday range of that week.
  • Week number → Date: enter a week year and week number to get all seven dates in that week.

ISO week numbering rules

Under ISO-8601:

  • The week starts on Monday.
  • Week 1 is the week that contains the first Thursday of the year.
  • Equivalently, week 1 is the week that contains January 4.
  • Most years have 52 weeks, but some have 53 weeks.

Key consequences of ISO week rules:

  • Dates at the very start of January can belong to the last ISO week of the previous year.
  • Dates at the very end of December can belong to week 1 of the next ISO week year.
  • The ISO week year shown by the calculator may differ from the calendar year of the date.

Algorithm used (technical details)

The calculator uses a standard algorithm equivalent to the one in ISO-8601 and many programming languages:

  1. Treat Monday as day 1 and Sunday as day 7.
  2. Shift the date to the Thursday of its week (this anchors the week to a single year).
  3. Compute the week year from that Thursday.
  4. Count how many weeks have passed since the first week of that week year.

This approach correctly handles leap years, 52/53-week years, and boundary dates around New Year.

Examples

  • 2025-01-01 (Wednesday) → ISO week 1 of 2025.
  • 2020-01-01 (Wednesday) → ISO week 1 of 2020.
  • 2015-01-01 (Thursday) → ISO week 1 of 2015, and 2015 has 53 weeks.
  • 2015-12-31 (Thursday) → ISO week 53 of 2015.

Common use cases

  • Project planning: align tasks and milestones to week numbers.
  • Timesheets & payroll: report hours by ISO week.
  • School or academic calendars: map teaching weeks to dates.
  • Software integration: match logs or reports that use ISO week numbers.

FAQ

How do I find the week number for a specific date?

Go to the Date to Week Number tab, pick your date, and the calculator will show:

  • The ISO week number (1–52 or 1–53).
  • The ISO week year.
  • The Monday–Sunday date range for that week.

Why does the week year differ from the calendar year?

ISO weeks are grouped into week years. If a week spans December–January, all seven days belong to the same week year. For example, if January 1 falls on a Friday, it may still be part of the last ISO week of the previous year.

How do I know if a year has 53 weeks?

A year has 53 ISO weeks if:

  • January 1 is a Thursday (or Wednesday in a leap year), or
  • December 31 is a Thursday.

The calculator automatically determines this and shows the valid week range for any year you enter.

Does this calculator support different week numbering systems?

This tool focuses on the ISO-8601 standard, which is the most widely used internationally. If you need a region-specific system (e.g., weeks starting on Sunday), you can still use the date range output to adapt it manually.