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ISO Week Number

ISO Week Number is part of Toolspire and is ready as soon as this page loads. Week 1 is the week that contains the first Thursday. 1 January is not always week 1. 1 January 2021 falls in week 53 of 2020.

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On this page

What you can do here

ISO year beside the week

A week number without its ISO year is ambiguous in the first and last days of December and January.

Thursday rule

The date is shifted to the Thursday of its week, and that Thursday’s year is the ISO year.

Not a US week

Weeks here do not start on Sunday, and week 1 is not simply the week of 1 January.

The sequence

How to use the ISO Week Number

  1. 1

    Use the written date

    The week is a property of the calendar date, not of the hour. Leave the time off.

  2. 2

    Read the year in the detail

    If you only copy the week number in early January, you may be quoting the previous ISO year.

  3. 3

    Check 1 January 2021

    The page should say week 53 and ISO year 2020.

Worth knowing

  • ISO Week Number on Toolspire: the ISO week starts on Monday. Week 1 is the week with the year’s first Thursday, which is also the week that contains 4 January.
  • 1 January 2021 is a Friday and belongs to ISO week 53 of 2020, not week 1 of 2021.
  • The headline is the week number. The detail is the ISO year, which can be the previous or next calendar year.
  • The date is read in UTC and checked the same way as the weekday tool, so 31 February is refused.

Why 1 January is not always week 1

ISO-8601 wants every week to have seven days and to start on Monday. That forces a few days of late December or early January into the neighbouring year. The practical test is the Thursday: whichever year owns the Thursday owns the week. This page moves the date to that Thursday, then counts weeks from 1 January of that year. It does not follow the “week of 1 January” rule some payroll calendars still use.

FAQ

Questions about ISO Week Number

Where does ISO Week Number run?

ISO Week Number runs in this tab on Toolspire. What you type or drop in is not sent to us and is not stored on a server.

What ISO week is 1 January 2021?

Week 53 of ISO year 2020. The Friday sits in the week whose Thursday was 31 December 2020.

When does week 1 start?

On the Monday of the week that contains the first Thursday of the calendar year. That Monday can fall in December.

Is this the same as a US calendar week?

No. Many US calendars start the week on Sunday and call the partial first week week 1. ISO does neither.

Can the ISO year differ from the date’s year?

Yes. That is why the detail prints the ISO year separately from the week number.

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