ISO 8601 weeks

Week Number Calculator

Find the ISO 8601 week number for any date, along with the week-year it belongs to, the Monday it starts on, the Sunday it ends on and how many weeks are left in the year.

—ISO week number
—ISO week-year
—Day of week
—Week starts (Monday)
—Week ends (Sunday)
—Day of the year
—Weeks left in the year
—ISO weeks this year
—US week (Sunday start)
ISO weekStarts (Mon)Ends (Sun)Week-year

Calculated in your browser using standard ISO 8601 rules.

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Correct ISO logic

Proper ISO 8601 rules, including the week-year that can differ from the calendar year.

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Full week context

See the Monday start, Sunday end and where the date sits within the year.

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Both conventions

The US Sunday-start number is shown alongside so cross-team confusion disappears.

How to use the week number calculator

Pick any date and the ISO week is worked out from the standard Monday-to-Sunday rule.

  1. Pick a dateUse the date field, or jump straight to today with one click.
  2. Read the ISO weekThe week number and its week-year appear together, since they can differ at year boundaries.
  3. Check the surrounding weeksThe table lists nearby weeks with their start and end dates for planning.

About ISO week numbers

ISO 8601 gives every week a number from 1 to 52 or 53, always running Monday to Sunday. Manufacturing, logistics and European business planning lean on it heavily, because a week number is unambiguous across languages and date formats. The one wrinkle is the year boundary, where the last days of December can belong to week 1 of the next year and the first days of January to week 52 or 53 of the last.

The Thursday rule

A week belongs to whichever year contains its Thursday. This is equivalent to saying week 1 always contains 4 January.

Week-year

Because of the Thursday rule, the year a week belongs to can differ from the calendar year of a given date within it.

53-week years

Roughly every five to six years the calendar produces 53 ISO weeks, which is a recurring headache for annual financial planning.

The US convention

In the United States, weeks generally start on Sunday and week 1 is simply the week containing 1 January, giving a different number for most dates.

Week number calculator FAQ

Questions about ISO rules, week-years and spreadsheet differences.

How does ISO 8601 define week 1?

Week 1 is the week containing the first Thursday of January, which is the same as the week containing 4 January. Weeks always run Monday to Sunday.

Why can a date in January belong to week 52 or 53 of the previous year?

If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the outgoing year under ISO rules. That is why the tool reports a week-year separately from the calendar year.

How many weeks are in a year?

Most years have 52 ISO weeks. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday.

Why does my spreadsheet give a different week number?

Spreadsheets default to a US convention where weeks start on Sunday and week 1 is simply the week containing 1 January. This tool shows both so you can see the difference.

Which countries use ISO week numbers?

Most of Europe plus much of Asia use them for business planning, logistics and manufacturing schedules. North America generally uses the Sunday-start convention instead.

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