Week Number Calculator

Find the ISO week number of any date, together with the first and last day of that week. Week numbers are widely used in Europe and in business planning, payroll and logistics.

The ISO 8601 rule

Under the international standard ISO 8601, weeks start on Monday, and week 1 of a year is the week that contains the year's first Thursday. Equivalently, it is the week that contains January 4. A year therefore has either 52 or 53 weeks.

Because of this rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, January 1, 2027 is a Friday, so it falls in week 53 of 2026.

Other week-numbering systems

In the United States, some calendars start weeks on Sunday and call the week containing January 1 "week 1". Results from that system can differ from ISO by one week. This calculator always uses ISO 8601, which is the default in Excel's ISOWEEKNUM function and in most European calendars.