Calendar Week Calculator
Find the calendar week for a date or the date for a calendar week. Quick, free, no registration required for instant date calculations.
Determine the calendar week (CW) according to ISO 8601 for any date.
Calendar week
KW 20 / 2026
Day of the week
Saturday
Week (Mon - Sun)
05/11/2026 - 05/17/2026
Day of the year
Day 136 of 365
Calendar weeks in May 2026
Note: These calculations are for informational purposes only and do not replace professional tax or financial advice. All information without guarantee.
Frequently Asked Questions
How are calendar weeks (KW) numbered?
According to ISO 8601, calendar week 1 is the week containing the first Thursday of the year. Weeks start on Monday. A year can have 52 or 53 calendar weeks.
Why does the calendar week sometimes differ between countries?
Most European countries use the ISO standard (Monday start, first week contains Jan 4). The US often uses a Sunday-start week where week 1 contains January 1, which can differ by one week.
What is the Calendar Week Calculator?
The calendar week calculator determines the calendar week (CW) of a date or shows all calendar weeks of a year with start and end dates.
How does the Calendar Week Calculator work?
Enter a date. The calculator shows the corresponding calendar week per ISO 8601. Alternatively enter a CW to get the start and end dates of that week.
Key Data and Facts
ISO 8601: CW starts on Monday. CW 1 contains the first Thursday of the year. A year has 52 or 53 calendar weeks.
Step-by-Step Guide
How to use den Kalenderwochen-Rechner step by step: 1. Datum enter -- der Rechner zeigt die zugehoerige Kalenderwoche (KW) nach ISO 8601 an. 2. Alternativ KW und year enter -- der Rechner zeigt den Montag und Sonntag dieser week. 3. ISO-8601-Regeln verstehen: Die week beginnt am Montag. KW 1 ist die week, die den ersten Donnerstag des Jahres enthaelt (oder anders formuliert: die week mit dem 4. Januar). 4. Besonderheiten am Jahreswechsel: Der 1. Januar kann in KW 52 oder 53 des Vorjahres fallen. Der 28.-31. Dezember kann in KW 1 des Folgejahres fallen. Ein year hat 52 oder 53 Kalenderwochen. 5. Haeufige Anwendungen: Terminplanung in business (Meetings in KW 15), Liefertermine (Lieferung in KW 20), Urlaubsplanung. Tipp: In Deutschland und den meisten europaeischen Laendern gilt ISO 8601. In den USA beginnt die week am Sonntag.
Calculation Example
10. April 2026 liegt in KW 15. KW 1 im year 2026 beginnt am Montag, 29. Dezember 2026, und endet am Sonntag, 4. Januar 2027.
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