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Week Number

This week's number is at the top, by the European ISO count. Change the date to look up any other week, or ask the other way round — which dates fall in week 36. Both the ISO and the American numbering are shown, because they disagree more often than people expect.

The number below is this week by the European (ISO) count. Change the date to look up any other week.

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Monday to Sunday
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Day of the week
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US week number
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Day of the year
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Weeks in this year
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Weeks left in the year
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Which dates fall in a given week

Everything is worked out in your browser and nothing is sent anywhere. The ISO rule is worth knowing: week 1 is the one containing the first Thursday of the year, so 1 January can belong to week 52 or 53 of the year before, and 29 December can already be week 1 of the next. That is also why a week has its own year, which is not always the calendar one.

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How to use

  1. The big number is the current week. «Today» brings the date back if you have changed it.
  2. Type or pick any date to see its week number, the Monday-to-Sunday range it belongs to, the weekday and the day of the year.
  3. To go the other way, put a year and a week number into the second block — you get the exact dates that week covers.
  4. If the two counts disagree for your date, a note says so and explains which is which. It appears only when there is a real difference.

Good to know

Week 1 is not simply the first seven days of January

By ISO 8601 — the rule used across Europe and in most business software — week 1 is the week containing the first Thursday of the year, which is the same as the week containing 4 January. Weeks run Monday to Sunday. The consequence surprises people: 1 January can belong to week 52 or 53 of the previous year, and 29 December can already be week 1 of the next one. Counting days from 1 January and dividing by seven gets the answer wrong by a whole week, and it does so around the turn of every other year — which is exactly when people are planning.

A week has its own year

Because weeks straddle the year boundary, a week number without a year is half an answer. 30 December 2019 is 2020-W01: written that way, it is unambiguous; written as «week 1», it invites the reader to assume 2019. This page always shows the week year next to the number, and it is the ISO year, which for a few days each January and December differs from the calendar year. Spreadsheets and project trackers do the same, which is why a date can appear under a year it does not belong to.

The American count is a different count, not a wrong one

In the United States the week starts on Sunday and week 1 is simply the week containing 1 January, so the first week is usually a partial one. That shifts almost every number by one against the ISO count, and it is a reliable source of confusion between colleagues in different countries: both are looking at «week 14» and meaning different seven days. Neither is wrong. Both are shown here so you can see which one your counterpart is using.

Frequently asked questions

What week of the year is it?

The number at the top of this page, by the ISO count used in Europe and in most calendars and business software. If you need the American numbering, it is in the list below the date, and a note appears whenever the two differ.

Why does my calendar show a different week number?

Almost always because it uses the other convention. Check what your calendar does with the first days of January: if it calls 1 January week 1 regardless of the weekday, it is using the US rule. Outlook, Google Calendar and iOS all have a setting for this.

How many weeks are in a year?

Fifty-two in most years and fifty-three in some — a year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. That happens roughly every five to six years; 2020 and 2026 are examples. The page shows the count for whichever year your date falls in.

Can I find out which dates are in a given week?

Yes, that is the second block. Put in the year and the week number and you get the Monday and the Sunday. If you ask for a week the year does not have — week 53 of a 52-week year — the page says so rather than quietly giving you the wrong dates.

Is my date sent anywhere?

No. Everything is worked out in this tab; nothing is sent to a server, stored or logged. The page works with the network switched off.

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