Week Number Calculator
Find the ISO 8601 week number for any date.
How this is calculated
Under ISO 8601, weeks run Monday through Sunday, and week 1 of a year is the week containing that year's first Thursday. Because of that rule, a date in late December can belong to week 1 of the following year, and a date in early January can belong to the last week of the previous year, this calculator handles both cases correctly rather than just using the calendar year.
Frequently asked questions
What week number is a given date?
Enter the date above and select Calculate. The result shows the ISO 8601 week number, which runs Monday through Sunday, with week 1 defined as the week containing the year's first Thursday.
Why does a date near January 1st sometimes show a week number from the previous or next year?
Under ISO 8601, a week belongs to whichever year contains that week's Thursday. So December 29-31 can fall in week 1 of the following year, and January 1-3 can fall in the last week (52 or 53) of the previous year, depending on which day of the week January 1st lands on.
Sources
- ISO 8601 standard, week-numbering rules (Monday-start weeks, first week contains the year's first Thursday)
Built and maintained by Ready Utilities, founded by Cedrick Reese, a retired veteran and web developer. Every formula and convention on this site is checked against real, current sources before publishing. This calculator runs entirely in your browser, nothing you enter is sent anywhere. Spot something that looks wrong? Let us know.