Week Number Calculator
Find the ISO 8601 week number and detailed info for any date.
This tool finds the ISO week number for any date - a standardized way of numbering weeks within a year (1 through 52 or 53), commonly used in business planning, project scheduling, and international contexts where "week 23" is clearer than a specific date range.
Frequently asked questions
It refers to ISO 8601, an international standard that defines weeks as starting on Monday, with week 1 being the week containing the year's first Thursday - this standardization avoids ambiguity between different regional week-numbering conventions.
52 weeks account for 364 days, one day short of a full year (365 or 366 days) - the leftover day(s) occasionally push the year into having a 53rd week under the ISO definition, depending on which day of the week the year starts and ends on.
Using the ISO 8601 standard specifically, yes - though some non-ISO conventions (like certain US business calendars) may number weeks slightly differently, which is exactly why using the standardized ISO version avoids confusion.