Week Number Calculator

The Week Number Calculator returns the ISO 8601 week number for any calendar date, answering questions like "what week of the year is this?" Under the ISO standard, weeks run Monday through Sunday, and week 1 is defined as the week containing the year's first Thursday — equivalently, the week containing January 4th. This means the ISO week number can occasionally differ from a naive count, and the first or last few days of a year may belong to week 52 or 53 of the adjacent year.

To use it, enter or pick a date and the tool reports the ISO week number, often alongside the ISO week-year (which can differ from the calendar year near January 1st). Because ISO weeks always start on Monday, the calculation finds the Thursday of the week containing your date, then counts how many such weeks have elapsed since the year began. This Thursday-anchored rule is what makes ISO week numbering unambiguous and consistent across systems.

Week numbers are widely used in business, manufacturing, payroll, and project planning, especially in Europe where references like "W23" or "KW23" (Kalenderwoche) are standard scheduling shorthand. Spreadsheets, ERP systems, and date libraries all expose ISO week functions, so this tool is handy for cross-checking a value or quickly converting a date to its planning-week label without opening a spreadsheet.

A practical tip: years can have 52 or 53 ISO weeks, and a year has 53 weeks when it starts on a Thursday or is a leap year starting on Wednesday. If a date near year-end shows a week number paired with next year's week-year, that is correct ISO behavior, not a bug. Pair this with a Leap Year Checker or Time Duration Calculator when you need to reason about how many planning weeks fall between two dates.