ISO Week Calculator

Find the ISO 8601 week number and week-year for any date.

How ISO week numbers work

ISO 8601 numbers weeks from 1 to 52 or 53. Weeks start on Monday, and week 1 of a year is defined as the week containing that year's first Thursday. In practice this means week 1 always contains January 4, and the last few days of December, or the first few of January, can belong to the week of the other calendar year.

Why the week-year is not always the calendar year

Because week 1 is anchored to the first Thursday rather than to January 1, a date can fall in a week that is numbered as part of the previous or next year. For example, January 1, 2016 was a Friday, so it falls in week 53 of week-year 2015, not week 1 of 2016. Similarly, December 31, 2018 was a Monday and belongs to week 1 of week-year 2019.

Worked example

January 1, 2021 was a Friday, which places it in week 53 of week-year 2020. The first Monday of week 1 in week-year 2021 is January 4, 2021.

Common questions

Enter today's date above and the result shows the current ISO week number and week-year.
Some calendars, especially in the United States, start the week on Sunday and number week 1 as the week containing January 1, regardless of which weekday that is. ISO 8601 always starts weeks on Monday and anchors week 1 to the first Thursday of the year, which can produce a different number for the same date.
Yes. A year has 53 ISO weeks when January 1 falls on a Thursday, or in a leap year when January 1 falls on a Wednesday. Most years have 52.
It is the year that owns the ISO week a date falls in, which is not always the same as the date's calendar year. A date in the last days of December or the first days of January is the most common place the two differ.

Continue with

All tools
Day of Week What day of the week any date falls on, past or future. Date Difference Days, weeks, months and years between two dates. Days Ago The exact date a given number of days ago. Weeks Between Dates Exact weeks and leftover days between two dates.