Current for 2026•As of: July 2026

Calendar Week Calculator Calculate ISO 8601 week number.

Calendar week for any date – Thursday rule, ISO year and quarter

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Calendar Week Calculator

Calculate the ISO 8601 calendar week for any date – instantly and free.

Please enter a date

Calendar week according to ISO 8601 – how the calculation works

The Thursday rule explained simply

The calendar week calculator calculates the ISO-8601-compliant calendar week for any given date. The decisive rule: week 1 is always the week that contains the first Thursday of the year. Since a week always starts on Monday, week 1 can already include days from the previous year – or January 1 can fall in the last week of the previous year.

In Germany, the calendar week is ubiquitous in business: production planning, delivery dates, vacation and project planning all use week numbers. Our calculator also shows the ISO year (which can differ from the calendar year) and the quarter.

Frequently asked questions about the calendar week

ISO 8601, the Thursday rule and special cases explained

Under ISO 8601, the week starts on Monday. Week 1 is the week that contains the first Thursday of the year. This means: if January 1 falls on a Friday, Saturday, or Sunday, it still belongs to the last calendar week of the previous year. Our calculator applies the Thursday rule correctly.

December 30, 2024, for example, falls in week 1 / ISO year 2025, even though it is still 2024. This always happens when December 31 falls on a Monday, Tuesday, or Wednesday – these days then belong to the first week of the following year.

No. Most years have 52 calendar weeks, some have 53. A year has 53 calendar weeks if January 1 falls on a Thursday, or if it is a leap year and January 1 falls on a Wednesday. For example, 2015 had a week 53.

The calendar week (KW in German) numbers the weeks of the year. It is commonly used for project planning, scheduling, production planning and payroll. "Delivery in week 25" is a common phrase in business.

For business

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