Find the ISO week number of a date

Find the ISO week number of a date. Handy for teams and individuals planning schedules, checking deadlines or preparing a document.

Find the ISO week number of a date. See a clear result update immediately in your browser, with no account, upload or stored data to worry about.

How to use

Enter the date in year-month-day form, for example 2025-03-14, in the field provided for the input.
Check that the month and day both use two digits, as an incomplete date is often rejected.
Start the calculation and read the week number displayed in the result area.
To test another day, replace the value and run the calculation again; no other setting is needed.
A small text file containing the date can be imported instead of typing the value by hand.

Example

The sample uses 30 December 2024, a date at the very end of the year. Pay attention to the year attached to the week number: near 31 December a day can belong to the last week of the current year or to the first week of the next one, which is exactly the confusion this tool removes.

Input

2024-12-30

Limits and formats

The tool follows the ISO definition, where weeks start on Monday, so counting systems that begin on Sunday will not match.
One date is handled per calculation; a whole series means repeating the entry or moving to a spreadsheet.
Dates written in another order, such as day then month, may be misread, so keep to year-month-day.
The output gives the week number only, not public holidays or the number of working days in a month.

Where processing happens

The calculation happens inside your browser using the date you type or import. Nothing is transmitted and no trace is left when you close the page, so you can check dates from confidential project schedules without worrying about where they go.

Frequently asked questions

Why does a late December date land in week 1?

ISO rules attach every day to the week containing its Thursday. That means a few days at the end of the year can belong to the first week of the following year, and the number you see reflects that.

Is Monday always the first day of the week here?

Yes, the calculation follows the ISO convention, where a week runs from Monday to Sunday. If your organisation starts counting on Sunday, the number you get will not line up with your own reports.

Can I also see the start and end date of that week?

The tool returns the week number for the date you entered and does not break the week down into individual days. For the full range, a calendar or a spreadsheet is the better place to look.