What Week Number Is It?

About ISO Week Numbers

The ISO 8601 standard defines a week as starting on Monday and ending on Sunday. Week 1 of any year is defined as the week containing the year's first Thursday. This rule means that January 1 can sometimes belong to Week 52 or 53 of the previous year — for example, if January 1 falls on a Friday, Saturday, or Sunday, the entire week belongs to the prior year since that week's Thursday is in the previous year.

ISO week numbering is the international business standard and is widely used in European countries, international logistics, project management software, and financial reporting. Many industries use week numbers instead of calendar dates for planning, since weeks provide a clean 7-day structure that doesn't split like months do.

How to Find the Week Number for Any Date

To find the ISO week number for any date, scroll to the week calendar above and locate the row where your target date falls between the Monday and Sunday columns. Click any week row to highlight it. The table covers all 52 or 53 weeks of the selected year. Use the year navigation arrows to switch between years.

Most spreadsheet software also supports week number functions: in Excel use WEEKNUM(date,2) for ISO week numbers (the second argument "2" specifies Monday as the week start). In Google Sheets use ISOWEEKNUM(date).

Which Countries Use ISO Week Numbers?

ISO week numbering is the official standard in most European countries including Germany, France, the Netherlands, Scandinavia, and the UK. It is also standard in Australia, New Zealand, and most international business contexts. In the United States, weeks traditionally start on Sunday and week numbering follows a different convention — many US calendar applications use Sunday-based week 1 rules where January 1 always begins week 1. When working across US and European teams, always clarify which week number standard is being used to avoid scheduling confusion.