Week Number Calculator

Find the ISO week number.

Select a date to see the ISO calendar week, ISO year, weekday, and week range.

How it works

The calculator uses the ISO week-date rule based on the Thursday of the selected week.

The output uses the Monday-to-Sunday ISO week calendar.

Practical examples

  • 2021-01-01 is Friday in ISO week 53 of ISO year 2020.
  • 2021-01-04 is Monday in ISO week 1 of ISO year 2021.
  • For 2026-07-21, the displayed ISO week range is Monday 2026-07-20 through Sunday 2026-07-26.

How to read the result

  • ISO weeks run from Monday through Sunday.
  • The ISO year is determined by the Thursday in the selected week and can differ from the calendar year near New Year.
  • The output also reports weekday, day of year, quarter, and the Monday-to-Sunday range.

Limits and privacy notes

  • This calculator follows ISO 8601 week rules, not locale-specific Sunday-first numbering.
  • Day and weekday names use the browser's local display settings.
  • The selected date is processed locally in your browser and is not uploaded.

Method used

  • The selected date is shifted to Thursday of its ISO week.
  • Week 1 is the week containing the first Thursday of the ISO year.
  • The week number is counted from the start of that ISO year, while the range is calculated from Monday to Sunday.
ISO rules

Monday-start weeks

  • ISO weeks start on Monday.
  • The ISO year can differ near New Year.
  • Week range shows Monday through Sunday.
Planning context

More than week number

  • Day of year helps with annual planning.
  • Quarter helps with business reporting.
  • Week range is easier to share than a number alone.
Locale note

Country rules differ

  • This page uses ISO week rules.
  • Some countries label weeks differently.
  • Use local standards for official reporting.

Frequently asked questions

Why can January 1 belong to the previous ISO year?

ISO week 1 is the week containing the year's first Thursday. The first days of January can therefore remain in the previous ISO week-year.

Do ISO weeks start on Sunday?

No. ISO 8601 weeks start on Monday and end on Sunday.

Why is the ISO year shown separately?

A date's calendar year and ISO week-year can differ around the end and beginning of a year.

Is this the same as every country's week number?

No. Some calendars and software use locale-specific Sunday-first or partial-week rules.

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