What week is it?

How ISO week numbers work

ISO 8601 is the international standard for writing dates. Its weeks always start on Monday and end on Sunday. Week 1 is the week that contains the year's first Thursday. Another way to say the same thing: week 1 is the week with January 4 in it.

Thursday is the middle of a Monday to Sunday week. So the rule gives each week to the year that holds most of its days. A week never belongs to two years, which is why the first days of January can be in week 52 or 53 of the previous year, and the last days of December can already be in week 1 of the next.

Why some years have 53 weeks

364 days make exactly 52 weeks. A year is one or two days longer, and those extra days add up. Every few years they push a 53rd Thursday into the calendar year, and the ISO year then has 53 weeks. That happens when January 1 is a Thursday, or when a leap year starts on a Wednesday.

2020 was a leap year that began on a Wednesday, so it had 53 weeks. 2026 begins on a Thursday and has 53 as well. After that the next one is 2032. Over a full 400-year cycle, 71 years get a 53rd week.

ISO week numbers vs US week numbers

In the US, calendars usually start the week on Sunday. Week 1 is simply the week that contains January 1, however few days of it fall in January. The two systems often land on the same number in the middle of the year and drift apart at the edges.

Take Sunday, January 3, 2021. In ISO numbering it is the last day of week 53 of 2020. In US numbering it starts week 2 of 2021. The US system can even reach week 54: December 31, 2028 is a Sunday and starts US week 54, while in ISO terms it is week 52. If a colleague quotes a week number, check which system their calendar uses.

How to show week numbers in your calendar

Google Calendar

On the web, open Settings with the gear icon. Under General, choose View options and tick Show week numbers. The numbers appear in the small month calendar and in Month and Year views. The Android and iPhone apps do not have this switch.

Outlook

In new Outlook and Outlook on the web, go to Settings, Calendar, View. Under Calendar appearance, turn on Show week numbers and save. They show in Month view. In classic Outlook for Windows the option is under File, Options, Calendar, Display options. To match ISO numbering, set the first day of the week to Monday and the first week of the year to the first 4-day week.

Excel and Google Sheets

=ISOWEEKNUM(A1) returns the ISO week of the date in cell A1. =WEEKNUM(A1,21) is the same thing. A plain =WEEKNUM(A1) gives the US style number, with Sunday as the first day of the week.

Planning by weeks works for a lifetime too. The life in weeks calculator puts every week you have lived on one grid, and the New Year countdown shows exactly how much of this year is left.

Questions people ask

What week of the year is it right now?

The large number at the top of this page is the ISO 8601 week for the date on your device, so it follows your own time zone. ISO weeks run from Monday to Sunday, and week 1 is the week that contains the first Thursday of the year.

How many weeks are in a year?

In the ISO system a year has 52 or 53 weeks. It gets 53 when January 1 falls on a Thursday, or when a leap year starts on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks. 2027 has 52, and the next 53-week year is 2032.

Why is January 1 sometimes in week 52 or 53?

ISO weeks never split across years. If January 1 falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year. January 1, 2027 is a Friday, so it sits in week 53 of 2026. Week 1 of 2027 starts on Monday, January 4.

Is the US week number different from the ISO week number?

Often, yes. Many US calendars start the week on Sunday and call the week with January 1 week 1, even if it has only one day. Sunday, January 3, 2021 is ISO week 53 of 2020 but US week 2 of 2021. Use the date lookup above to compare both for any date.

How do I get the ISO week number in Excel or Google Sheets?

Use =ISOWEEKNUM(A1), where A1 holds a date. =WEEKNUM(A1,21) gives the same result. Plain =WEEKNUM(A1) uses the US style instead: weeks start on Sunday and the week with January 1 is week 1.