Week Number Calculator
Enter a date to get its ISO 8601 week number and the Monday and Sunday that begin and end that week.
What is a week number?
A week number labels each week of the year with a number from 1 to 52, or 53 in some years. Offices, factories, schools and software teams use them to plan work: it is quicker to say “delivery in week 14” than to give a full date range. The most widely used system is ISO 8601, the international standard for writing dates, and that is the system this calculator follows.
Under ISO 8601 every week starts on Monday and ends on Sunday, and week 1 is the week that contains the year’s first Thursday. Because of that rule, a few days at the start of January can belong to the last week of the previous year, and a few days at the end of December can belong to week 1 of the next year. The calculator shows the ISO year next to the week number so this is always clear.
How to use it
- Choose any date.
- Press “Find Week Number”.
- Read the week in the standard form (for example 2026-W41), the number of weeks in that ISO year, and the Monday and Sunday that bound the week.
To find the dates of a known week number, enter any date you expect to be inside it and adjust by seven days until the week matches.
Worked examples
Example 1: a date in mid-October
Example 2: weeks that cross the new year
Frequently asked questions
What is an ISO week number?
It is the week numbering defined by the ISO 8601 standard. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year.
Why does 1 January sometimes show week 52 or 53?
If 1 January falls on a Friday, Saturday or Sunday, that week's Thursday belongs to the previous year, so the whole week counts as the last week of the previous year.
How many weeks does a year have?
Most ISO years have 52 weeks. A year has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2020 and 2026 are 53-week years.
Do US calendars use the same numbering?
Not always. Some US calendars start weeks on Sunday and call the week containing 1 January week 1. This tool uses the ISO system, which is common in Europe and in business software.
Related tools
To get the weekday and day of the year for a date, use the Day of the Week calculator.
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