ISO Week Number Calculator

The ISO Week Number Calculator tells you which ISO 8601 week a calendar date falls in. Pick any date and you instantly see its ISO week number (a value from 1 to 53), its ISO week-year, the combined YYYY-Www label (for example 2020-W25), the weekday, and the Monday–Sunday span of that week. It also tells you whether the ISO year is a long one with 53 weeks.

Everything runs entirely in your browser — the date you enter is never uploaded anywhere.

How to use it

1. Choose a Date with the date picker (or type it as YYYY-MM-DD).
2. Read off the result table: ISO week number, ISO week-year, the YYYY-Www label, the day of the week, and the first (Monday) and last (Sunday) day of that week.
3. Copy the table or download it as a CSV.

The result updates automatically as soon as you change the date.

What the ISO 8601 week rule is

The ISO 8601 standard defines weeks precisely so that every date belongs to exactly one week, worldwide, with no ambiguity:

• Weeks start on Monday and end on Sunday.
• Week 1 is the week that contains the year's first Thursday — equivalently, the week containing January 4th. That means week 1 is always the first week with the majority of its days (four or more) in the new year.
• Because a week can straddle New Year's Eve, a date's ISO week-year can differ from its calendar year. The last few days of December may belong to week 1 of the following year, and the first few days of January may belong to week 52 or 53 of the previous year.

Why can the year be different from the calendar year?

Since a whole week is assigned to whichever year holds its Thursday, the boundary days get pulled across. For example, December 31, 2018 (a Monday) sits in the same week as Thursday January 3, 2019, so it is week 1 of 2019. Conversely, January 1, 2017 (a Sunday) shares its week with Thursday December 29, 2016, so it is week 52 of 2016. This is expected behaviour, not a bug — the ISO week-year field shows you the correct year to pair with the week number.

When does a year have 53 weeks?

Most ISO years have 52 weeks. A year has a 53rd week when it either starts on a Thursday, or is a leap year that starts on a Wednesday. For instance, 2020 is a long year: December 31, 2020 is a Thursday and lands in week 53.

Which dates are valid?

Any real calendar date works, including leap days such as February 29. Impossible dates (like February 30) are rejected. All calculations are done in UTC, so the result never shifts because of your local time zone.

Privacy

This calculator is fully client-side. The date you enter is processed on your own device and is never sent to a server, logged, or stored.