Calendar Week Calculator
Find the ISO 8601 calendar week of any date, with its weekday, the Monday-to-Sunday date range, the day of the year and all calendar weeks of that month.
Determine the calendar week (CW) according to ISO 8601 for any date.
Calendar week
Week 41 / 2026
Day of the week
Monday
Week (Mon - Sun)
10/05/2026 - 10/11/2026
Day of the year
Day 278 of 365
Calendar weeks in October 2026
Note: These calculations are for informational purposes only and do not replace professional tax or financial advice. All information without guarantee.
FAQ
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.
Why does 1 January sometimes fall in a week of the previous year?
The calculator shows the calendar week together with its ISO 8601 week-numbering year, e.g. "KW 53 / 2026". If 1 January falls on a Friday, Saturday or Sunday, it still belongs to the last week of the previous year: 1 January 2027 is a Friday and falls in KW 53 / 2026, which runs from 28 December 2026 to 3 January 2027. Conversely, the last days of December can already count as week 1 of the following year: KW 1 / 2026 began on Monday, 29 December 2025.
Guide
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 the calendar week calculator step by step: 1. Enter a date – the calculator shows the corresponding calendar week (CW) according to ISO 8601. 2. Alternatively enter the CW and year – the calculator shows the Monday and Sunday of that week. 3. Understand the ISO 8601 rules: the week begins on Monday. CW 1 is the week that contains the first Thursday of the year (or, put differently: the week with 4 January). 4. Special features at the turn of the year: 1 January can fall in CW 52 or 53 of the previous year. 28-31 December can fall in CW 1 of the following year. A year has 52 or 53 calendar weeks. 5. Common applications: scheduling in companies (meetings in CW 15), delivery dates (delivery in CW 20), holiday planning. Tip: in Germany and most European countries ISO 8601 applies. In the USA the week begins on Sunday.
Calculation Example
April 10, 2026 falls in week 15. Week 1 of 2026 begins on Monday, December 29, 2025, and ends on Sunday, January 4, 2026.
Sources
Official sources
Calculations are based on applicable German laws and official data:
Full methodology at Methodology.
Reviewed by Konstantin Iakovlev · Last updated:
Date & Time
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