Week Number Calculator
Enter any date to find its ISO 8601 week number.
Find Your Week Number
How ISO Week Numbers Work
ISO 8601 weeks start on Monday, and week 1 is defined as the week containing the year's first Thursday — which means the first few days of January can sometimes belong to the final week of the previous year, and the last few days of December can belong to week 1 of the next year.
Frequently Asked Questions
What is an ISO week number?
An ISO week number identifies which week of the year a date falls in, numbered 1 through 52 or 53, following the international ISO 8601 standard where weeks start on Monday.
How do I find the week number for a specific date?
Enter any date into the calculator and it instantly shows that date's ISO week number.
Do ISO week numbers start on Monday or Sunday?
ISO 8601 week numbers start on Monday. This differs from some calendars that treat Sunday as the first day of the week.