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What Week Number Is It?

Find the ISO week number for today or any date. See the week-year, day of the week, and the full Monday-to-Sunday range.

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ISO week number

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Monday – Sunday
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Day of the week
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ISO week-year
Runs in your browser – your data never leaves your device Also available via our free API

How to use the Week Number Calculator

  1. 1 Choose a date, or keep today’s date selected.
  2. 2 Read the ISO week number and week-year shown above.
  3. 3 Check the Monday-to-Sunday date range and weekday for your selected date.

How week numbers are counted

This calculator uses the ISO 8601 week date system. Weeks start on Monday. ISO week 1 is the first week of the year that contains a Thursday (equivalently, 4 January). Most years have 52 ISO weeks; some have 53.

The week-year can differ from the calendar year

The first days of January can belong to the final ISO week of the previous year, and the last days of December can belong to week 1 of the next ISO year. For example, 1 January 2021 was in week 53 of ISO week-year 2020. This calculator shows the week-year alongside the week number to make that distinction clear.

Find a week number for any date

Choose a date to see its ISO week number, week-year, weekday and the week’s Monday-to-Sunday date range. Use it to plan work schedules, school terms, deliveries, projects and recurring events, or to compare week numbers across calendars.

ISO weeks and other systems

Some countries and calendars count weeks differently, such as starting on Sunday or defining week 1 as the week containing 1 January. If you need to match a local payroll or business calendar, check which rule it uses. The result here always follows ISO 8601.

Frequently asked questions

What week number is it today?

The date field starts with today’s date. The calculator shows its ISO week number and ISO week-year.

What is an ISO week number?

It is a week number under ISO 8601. Weeks begin on Monday, and week 1 is the first week containing a Thursday.

Can a week number belong to a different year?

Yes. Dates near New Year can belong to the previous or next ISO week-year. The calculator displays the week-year separately.

Does every year have 52 weeks?

Most ISO week-years have 52 weeks; some have 53, depending on how the calendar falls.

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