Week Number Calculator

Find the ISO week number for any date.

Last updated: 20 Aug 2026

Quick answer

This gives the ISO-8601 week number for a date (weeks start on Monday; week 1 contains the first Thursday of the year). For example, 1 January 2025 falls in ISO week 1. Enter a date above.

Date

Find the ISO week number for any date. ISO-8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday.

How to find the week number of a date

1 Enter a date.
2 We compute its ISO-8601 week number.
3 You see the week number and its year.

Frequently asked questions

What is an ISO week number?

A standard way to number weeks 1–52/53, starting Monday, where week 1 holds the first Thursday of the year.

Why can early January be week 52/53?

Under ISO rules, the first days of January can belong to the last week of the previous year.

What week is it now?

Enter today's date to see the current ISO week number.

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