Week number
What week is it? The ISO week number of any date, the dates of any week, every week of a year, and the US numbering.
How to use
The page opens on today: the ISO week number, the Monday and the Sunday it runs from and to, the day of the year, the days left after today, and how many weeks the year has. Choose another date to see its week; choose The dates of a week to go from a week number to its seven days, or Every week of a year for a table of all of them, which can be copied or downloaded as a CSV file.
ISO 8601, the international standard used across Europe and by most calendars outside North America, starts weeks on Monday, and week 1 is the week that contains the year’s first Thursday, which is the same as the week that contains 4 January. Every week therefore has seven days, a year has 52 or 53 weeks, and the first days of January can belong to the last week of the year before: 1 January 2027 is in week 53 of 2026. The years with 53 weeks are those that start on a Thursday, and leap years that start on a Wednesday: 71 of every 400 years.
The United States numbering, which is also the default of Excel’s WEEKNUM, starts weeks on Sunday and counts the week that contains 1 January as week 1, however few of its days fall in January. Its first and last weeks are therefore usually partial, a year has 53 weeks, and a leap year that starts on a Saturday has 54: 31 December 2028 is in week 54. The two systems often agree, but not always, so check which one a calendar, a client or a spreadsheet uses before comparing week numbers.
Dates are counted as calendar days, so time zones and clock changes never move a date into another week. In a spreadsheet, =ISOWEEKNUM(A1) gives the ISO week in Excel and Google Sheets, and =WEEKNUM(A1) the US one. Everything is worked out in your browser.
Example
30 October 2026ISO week 44, from Monday 26 October to Sunday 1 November 2026; day 303 of 365, with 62 days left. It is week 44 in the US system too.1 January 2027ISO week 53 of 2026, because the Thursday of that week, 31 December, is in 2026. In the US system it is week 1 of 2027.Week 1 of 2026 (ISO)Monday 29 December 2025 to Sunday 4 January 2026: the week that contains 1 January 2026, a Thursday.31 December 2028US week 54, because 2028 is a leap year that starts on a Saturday. In ISO numbering it is week 52 of 2028.
Frequently asked questions
What week is it today?
The page shows it as soon as it opens, from your device’s date: the ISO week, its dates and the US number.
Why does 1 January sometimes belong to week 52 or 53?
In ISO 8601 a week belongs to the year that holds its Thursday. When 1 January falls on a Friday, Saturday or Sunday, the Thursday of its week is in December, so the week is the last one of the previous year.
How many weeks does a year have?
In ISO 8601, 52 or 53: 53 when the year starts on a Thursday, or on a Wednesday in a leap year. 2026 has 53 and 2027 has 52. In the US system a year has 53, or 54 in a leap year that starts on a Saturday.
Why do two calendars show different week numbers?
Because they use different systems. ISO weeks start on Monday and count as week 1 only with four or more days in the new year; US weeks start on Sunday and week 1 is whichever week holds 1 January. On 3 January 2021 the ISO week was 53 of 2020 and the US week was 2.
How do I get the week number in Excel or Google Sheets?
=ISOWEEKNUM(date) gives the ISO week and =WEEKNUM(date) the US week, which starts on Sunday. In Excel, =WEEKNUM(date,21) also gives the ISO week.