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What Week Is It? – ISO Week Number Calculator

Find the current week number and the week number of any date (ISO 8601 or US system), with the start and end dates of every week of the year.

  • Free, no sign-up
  • Runs in your browser, nothing uploaded

About the Week Number

See today's week number and look up the week of any date. Choose ISO 8601 weeks (Monday start, used in Europe and most of Asia) or US weeks (Sunday start), with a table of all the weeks in the year.

Key features

  • Today's week number at a glance
  • Week number for any date
  • ISO 8601 (Monday start) or US (Sunday start)
  • Start and end date of the week
  • Every week of the year in a table
  • ISO week date notation (2026-W43-5)

What week is it?

Week numbers are widely used for planning, payroll, school terms and production schedules. See today's week number instantly, or look up any date.

ISO vs US week numbers

ISO 8601US system
Week startsMondaySunday
Week 1Contains the first ThursdayContains 1 January
Weeks per year52 or 5353 or 54 (partial weeks)
Used inEurope, much of AsiaUS, Canada

ISO week date format

2026-W43-5 means year 2026, week 43, day 5 (Friday). It's useful for unambiguous references in spreadsheets and software.

Day of the week calculator, working days calculator, and add days to a date.

How to use the Week Number

  1. Pick a date, a time or the cities you want; your own time zone is filled in for you.
  2. Search for any city or time-zone name such as EST, GMT or PKT.
  3. Results update instantly as you change anything.
  4. Your city lists are remembered in this browser for next time.

Frequently asked questions

How are ISO week numbers calculated?

ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing 4 January). Most years have 52 weeks; some have 53.

Why is 1 January sometimes in week 52 or 53?

If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last ISO week of the previous year. For example, 1 January 2027 is in week 53 of 2026.

Which countries use ISO week numbers?

Most of Europe and much of Asia use ISO 8601 weeks, which is common in business planning and manufacturing. The US and Canada usually count weeks starting on Sunday, with week 1 containing 1 January.

Which years have 53 ISO weeks?

Years that start on a Thursday, and leap years that start on a Wednesday. 2026 and 2032 have 53 weeks.

Do these tools handle daylight saving time?

Yes. Conversions use your browser's up-to-date time-zone database, so clock changes in the US, Europe and elsewhere are applied for the exact date you choose.

Which cities can I search?

Around 2,800 cities worldwide, plus common time-zone names such as UTC, GMT, EST, PST, CET, IST, PKT and GST.

Is anything saved?

Your chosen city lists are remembered in your own browser so they're ready next time. Nothing is sent to a server.

Last updated:

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