Week Number Calculator

Find the ISO week number of any date, the week’s start and end dates, and how many weeks the year has.

What week number is it today?

Leave the date empty for today or pick any date. The calculator returns the ISO 8601 week number (weeks start on Monday and week 1 contains the year’s first Thursday), the week’s start and end dates, and the US-style week number that starts on Sunday.

ISO week rules

Weeks run Monday to Sunday.
Week 1 is the week with the first Thursday of the year, so 1–3 January can fall in week 52 or 53 of the previous year.
A year has 53 ISO weeks when it starts on a Thursday (or a Wednesday in a leap year).

Frequently asked questions

Why do some calendars show a different week?

The US often counts weeks from Sunday and from 1 January; the tool shows both.

What is ISO week date format?

YYYY-Www-D, e.g. 2026-W41-3 for Wednesday of week 41.

Is it free?

Yes.