Week Number Calculator
Enter a date to get its ISO 8601 week number (Kalenderwoche / KW), its day of the year, and the days remaining — the figures German businesses and logistics teams rely on every day.
Weeks start on Monday
ISO 8601 defines Monday as the first day of the week, so a date's week number may differ from calendars that start weeks on Sunday.
Early January or late December?
Dates around New Year can belong to a different year's week count — for example, 1 January 2021 is in KW 53 of 2020, not KW 1 of 2021.
What is ISO week number?
The international standard for numbering calendar weeks
The ISO 8601 week number — abbreviated KW (Kalenderwoche) in German — is a way of counting the weeks within a year from 1 upward. Each week runs from Monday to Sunday, and week 1 is defined as the week that contains the first Thursday of the year. In German business, logistics, publishing, and project planning, meetings and deadlines are routinely scheduled by KW rather than by calendar date, making this an everyday figure in professional life.
Enter any date to get its ISO week number (KW), its ordinal day within the year, and the number of days still remaining before 31 December.
The ISO algorithm locates the Thursday of the week that contains your date — because the week is identified by the year its Thursday falls in — then counts how many seven-day periods separate that Thursday from the first Thursday of the year.
ISO Week = 1 + round((Thursday of week − First Thursday of year) ÷ 7 days)For example, 17 June 2026 is a Wednesday. Its week's Thursday is 18 June 2026. The first Thursday of 2026 is 1 January 2026. The gap is 168 days, and 168 ÷ 7 = 24 complete weeks, so the week number is 1 + 24 = KW 25. The ordinal day of the year is computed separately: count the days elapsed since 1 January and add 1, giving day 168 of 365. Days remaining = 365 − 168 = 197.
The ISO week number tells you at a glance where a date sits within the business calendar. KW 1–13 cover the first quarter, KW 14–26 the second, KW 27–39 the third, and KW 40–52 (or 53) the fourth. A typical year has 52 weeks; years whose 1 January or 31 December is a Thursday have 53 weeks — this happens roughly every five to six years (2015, 2020, 2026, 2032). If your result shows KW 52 or KW 53 for a date in early January, or KW 1 for a date in late December, that is correct: the date belongs to the neighbouring year's week count by the ISO rule. The day-of-year figure (1–365 or 1–366 in a leap year) is useful for progress tracking — day 168 of 365 means roughly 46% of the year has passed. The days-remaining figure counts calendar days from your date to 31 December inclusive; at KW 25 on 17 June 2026 there are 197 days left, enough time to complete two full quarters of annual targets before year-end.
ISO week numbering is a calendar convention, not a legal or financial standard in most contexts.
ISO week ≠ Sunday-based US week
Many American calendars and spreadsheet functions (Excel's WEEKNUM with mode
- start weeks on Sunday and define week 1 as the week containing 1 January, producing different results. Always confirm which convention your counterpart is using before scheduling cross-border meetings by week number. This calculator follows ISO 8601 (Monday start, Thursday rule) exclusively.