Calendar Week Calculator
Find the ISO calendar week for any given date, or determine the exact range of a week, directly in your web browser. Runs in your browser, GDPR-compliant.
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How to use this tool (video)
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Calendar Week Calculator: Week Number from Date or Date Range from Week
This calendar week calculator follows the international ISO 8601 standard and works in both directions: the calendar week for a date, or the full date range (Monday to Sunday) for a calendar week. It also shows the weekday and the ISO year. A calendar week always runs Monday to Sunday, and some years (e.g. 2020) have 53 weeks.
How the ISO calendar week works
The ISO 8601 standard defines how calendar weeks are counted. Two rules matter most:
- Week 01 is the week that contains the first Thursday of the year; in other words, the week into which 4 January falls.
- Every week runs from Monday to Sunday.
- A year has 52 or 53 calendar weeks (for example 2020 had 53).
- The turn of the year can fall in the middle of a week; that week then belongs to the ISO year that holds most of its days.
The calculator always returns the ISO week number, not the US convention, in which the week starts on Sunday and different rules apply.
Worked examples with real numbers:
- 1 January 2021 (a Friday) belongs to week 53 of ISO year 2020, because that week lies mostly in December 2020.
- 4 January 2021 (a Monday) starts week 1 of the year 2021.
- 15 August 2026 falls in week 33 of the year 2026.
What the calculator can do
- Find the ISO 8601 calendar week for a date
- Show the date range from Monday to Sunday for a calendar week
- Output the weekday and ISO year as well
- Detect invalid weeks (for example week 53 in a 52-week year)
What it can and cannot do
The calculator applies the ISO 8601 rules consistently, so it fits Germany, Austria and Switzerland and most European scheduling and planning systems. For US week numbering it deliberately gives different results, because there the week starts on Sunday. It makes no workplace or legal statements about whether a week counts under a collective or employment agreement; for binding decisions check the relevant rule.
Why the calculator matters
Calendar weeks appear in project plans, deadlines, release dates, delivery dates and agreements (due by week 34). The quick conversion between a date and a week is also useful for holiday and shift planning or in schools (which week do the holidays fall in?). Because the calculator runs entirely in your browser, you can safely enter private appointments.
Frequently asked questions
Which week is it right now?
The calculator pre-fills the current date. One click on Calculate shows the current calendar week and ISO year.
Why do some years have 53 weeks?
A year has 53 calendar weeks when January 1 falls on a Thursday. The first week then contains at least four days of the new year.
What does ISO year mean?
The ISO year can differ from the calendar year. When the turn of the year falls in the middle of a week, that week is assigned to the year holding most of its days. So 31 December can already belong to week 1 of the following year.
Why does the number differ from the US week count?
In the US the week starts on Sunday and different rules define the first week of the year. The ISO 8601 standard used here starts the week on Monday and refers to the first Thursday. For European dates the ISO count is the binding one.
Are my data stored?
No. All calculations run entirely in your browser. Your input is not transferred to or stored on any server.