Why might the week-year differ from the calendar year?

Under ISO 8601, a date in early January can belong to the last week of the previous year, and a date in late December can belong to week 1 of the following year, because weeks always run Monday through Sunday.

Do all years have 52 weeks?

No — most ISO years have 52 weeks, but years whose January 1 falls on a Thursday (or leap years starting on a Wednesday) have 53.

Age, Date & Calendar

Week Number Calculator

Enter a date to find its ISO 8601 week number.

Your calculation is performed in your browser.

Enter a date to find its ISO week number.

How it works

The calculator uses the ISO 8601 standard, where weeks run Monday to Sunday and week 1 of a year is defined as the week containing that year's first Thursday.

Formula & methodology

Each date is shifted to the Thursday of its own Monday–Sunday week; the ISO week number is that Thursday's position, counted in whole weeks, from January 1 of its year.

Worked example

January 1, 2027 falls in ISO week 53 of 2026, since it belongs to the Monday–Sunday week that started in late December.

Important notes

—Because ISO weeks start on Monday and are anchored to the first Thursday of the year, dates in very early January or very late December can belong to the week-year before or after the calendar year — this is expected and is called out in the result.
—Most years have 52 ISO weeks; some have 53, which the result shows automatically.