Week Number Calculator
Find the ISO 8601 week number for any date. See the current week number, week start/end dates, and convert between week numbers and calendar dates.
Built by Michael Lip
Frequently Asked Questions
What is the ISO 8601 week number?
ISO 8601 defines weeks as starting on Monday. Week 1 is the week containing the first Thursday of the year (or equivalently, the week containing January 4th). A year has either 52 or 53 ISO weeks. The format is YYYY-Www (e.g., 2026-W15).
Can January 1 be in week 52 or 53?
Yes. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year. For example, January 1, 2027 is a Friday, so it is part of 2026-W53. This is one of the most common gotchas with ISO week numbering.
How do I get the week number in JavaScript?
JavaScript has no built-in ISO week number function. You need to calculate it: find the nearest Thursday (ISO weeks are defined by Thursdays), then compute the number of days from January 1 of that Thursday's year, and divide by 7. Libraries like date-fns provide getISOWeek() for this.
What is the difference between ISO weeks and US weeks?
ISO 8601 weeks start on Monday. US convention starts weeks on Sunday. The US does not use ISO week numbering in business contexts — instead, it typically numbers weeks starting from the first Sunday of the year. The two systems can disagree by up to a week.
Is this tool free?
Yes. All EpochPilot tools are free, run in your browser, and require no signup or account.
How It Works
This calculator implements the ISO 8601 week numbering algorithm in pure JavaScript. Given any date, it finds the nearest Thursday to determine the ISO year, then counts the weeks from January 1 of that ISO year. The result is the standard YYYY-Www format used in international business, logistics, and software development.
ISO week numbers are widely used in European business, sprint planning (Agile), fiscal reporting, and data analytics. Many BI tools, ERP systems, and EU government reports reference weeks by their ISO number. If you need to convert between "Week 15" in a spreadsheet and actual calendar dates, this tool does it instantly.
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Privacy
Everything runs locally in your browser. No data is transmitted to any server. Source code is open on GitHub.
Contact
EpochPilot is built by Michael Lip. Email [email protected] for feedback.