Week Number Calculator

The week number calculator tells you which week of the year any date falls in, using the ISO 8601 standard. Open the page and it fills in today's date and shows the current week number straight away; enter any other date to see its week instead.

Week numbers are used across business, manufacturing, logistics, and project planning — schedules, delivery plans, payroll runs, and academic and fiscal calendars are often organised by "week 32" rather than a specific date. Knowing the current week number, or the week a particular date sits in, keeps everyone working to the same reference point.

This calculator follows the ISO 8601 rules, which are the international standard and the ones used across Europe and by most software. ISO weeks always start on Monday, and week 1 is the week that contains the year's first Thursday — equivalently, the week containing 4 January. A year has either 52 or 53 ISO weeks. Because of these rules, the first few 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. The calculator shows the ISO week-year alongside the week number so this is always clear.

As well as the week number, the result shows the weekday and the day of the year, giving you a full picture of where the date sits. Everything is computed privately in your browser, anchored to UTC so the answer never shifts with your time zone.

To use it, read today's week number on load, or choose a date and press Find the week number. To count the days or weeks between two dates instead, the days between dates calculator does that, and the day of the week calculator names the weekday for any date.

The calculator applies the ISO 8601 rule: weeks start on Monday and week 1 is the week containing the year's first Thursday (the week that includes 4 January). It finds the Thursday of the date's own week, compares it with the first Thursday of that ISO year, and counts the weeks between. The ISO week-year is read from that Thursday, so dates near the new year map to the correct year.