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- ISO weeks start on Monday and week 1 is the week that contains January 4.
- US weeks start on Sunday and week 1 is the week that contains January 1.
- Near New Year a date can belong to week 52 or 53 of the old year or week 1 of the next.
- A year has 52 or 53 ISO weeks.
How to use the week number calculator
Pick a date
Choose any date. It starts on today.
Choose the system
Pick ISO (Monday start) or US (Sunday start).
Read the week
See the week number, the first and last day of the week and the week-year.
Check the extras
See the weeks in the year, weeks left, the day of the year and the number in the other system.
How week numbers work
There are two common systems. ISO 8601 is used in most of the world and in business calendars. The US style is common in spreadsheets and US calendars. They can give different numbers for the same date.
Week number rules
ISO 8601: weeks run Monday to Sunday and week 1 contains January 4.
US style: weeks run Sunday to Saturday and week 1 contains January 1.
ISO week = the week of the Thursday in that week, counted from the first Thursday of the year.
The ISO week-year can differ from the calendar year for dates in late December and early January.
Week number examples
Example 1: 6 November 2026
A Friday.
- ISO week 45 (Nov 2 to Nov 8)
- US week 45 (Nov 1 to Nov 7)
- Day of the year 310
Both systems agree in this case.
Example 2: 1 January 2021
A Friday at the start of the year.
- ISO week 53 of 2020
- US week 1
ISO treats it as the last week of 2020 because that week has only three days in 2021.
Example 3: 30 December 2024
A Monday near the end of the year.
- ISO week 1 of 2025
- US week 53
The ISO week-year is already 2025.
Years with 53 ISO weeks
Recent and coming years.
- Years 2004, 2009, 2015, 2020, 2026, 2032, 2037, 2043
2026 has 53 ISO weeks and 53 US weeks.
Where a week number calculator helps
Plan projects
Use week numbers in schedules and sprints.
Read shipping dates
Many suppliers quote delivery by week number.
Fill in reports
Match the week column in a spreadsheet.
Check the year end
See whether late December belongs to week 1.
If you work with Europe or a business calendar, use ISO. If you follow a US calendar or a spreadsheet that starts weeks on Sunday, use US. Related tools: Age Calculator, Working Days Calculator and Date Difference Calculator.
Week number calculator FAQ
What week number is it today?
Today is on the date field by default, so the tool shows the current week in the system you choose.
What is the ISO week number?
It is the week number under ISO 8601, where weeks start on Monday and week 1 contains January 4.
Why does 1 January sometimes fall in week 52 or 53?
In ISO, week 1 must have at least four days in the new year. If it does not, those days belong to the last week of the old year.
How many weeks are in a year?
Either 52 or 53 under ISO. 2026 has 53.
Why are the ISO and US week numbers different?
They start the week on different days and define week 1 differently.
What is the week-year?
It is the year that the week belongs to. For dates near New Year it can differ from the calendar year.