ISO Week Number
Find the ISO calendar week of a date, making it easier to navigate the yearly calendar and plan around deadlines. Runs locally in your browser, GDPR-compliant.
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How to use this tool (video)
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ISO Week Number
The week-of-year calculator determines the ISO calendar week for any date. According to the ISO standard, each week begins on Monday and the first week of a year is the one that contains the first Thursday. This tool gives you the correct week number according to this international standard.
How it works
The ISO 8601 standard strictly defines calendar week numbering. Each week starts on Monday and ends on Sunday. The first week of a year is the one that contains January 4, meaning it has at least four days in the new year. In practice this means that week 1 of a year sometimes already includes days from the end of the previous year, and January 1 can fall in week 52 or 52 of the previous year. For a concrete example: January 1, 2025 is a Wednesday. According to ISO rules, this date falls in the last week of 2024, that is week 52, because the first full week of 2025 starts on Monday December 30, 2024, and January 4, 2025 is the first Thursday of the new year. January 6, 2025, a Monday, is the start of week 2 of 2025.
What the tool can and cannot do
The week-of-year calculator shows the correct ISO calendar week for any valid date. It automatically detects year transitions and returns a week number from 1 to 53. The tool cannot process other calendar systems and is limited to the ISO 8601 standard. It does not calculate seasons or quarters and does not display holidays. It is a pure week-number utility based on the ISO standard.
Why local processing in the browser
All calculations happen directly in your browser. Your date inputs are never sent to a server. There is no upload, no server log files, and no access by third parties. You need no account and no registration. After the page loads, the calculator works offline too because all resources are cached locally. There is no server that could be attacked and your data is not transmitted over the network.
Frequently asked questions
What is the ISO calendar week?
The ISO calendar week according to ISO 8601 begins on Monday. The first week of a year is the one containing January 4 and thus has at least four days in the new year. Week numbers range from 1 to 53.
Do I need to be online?
No, after the page has loaded once, the calculator works offline. The required resources are cached in the browser and a flaky connection will not interrupt you.
Are my data uploaded?
No, your date inputs are never sent to a server. Everything is processed locally in your browser with no upload and no third party access.
Can January 1 fall in week 52?
Yes, when January 1 is a Tuesday, Wednesday, Thursday, or Friday, it can fall in week 52 of the previous year because the first full week of the new year has not yet begun.
How many weeks does a year have?
A normal year has 52 weeks and one extra day. A leap year has 52 weeks and two extra days. This means a week can number 53 when the extra days form a complete week.
Is the tool free to use?
Yes, the week-of-year calculator is a free tool that runs directly in your browser. There are no hidden costs or limitations.