Week Number Calculator
Find the ISO 8601 week number for any date — which week of the year does any date fall in?
What Does This Tool Do?
Calculates the ISO 8601 week number (1-53) for any given date. Also shows the dates of the Monday and Sunday of that week.
Key Features
ISO 8601
Uses the international ISO 8601 standard (weeks start Monday).
Week Dates
Shows Monday and Sunday of the given week.
Instant
Result appears immediately.
How to Use This Tool
- Select a date.
- See the ISO week number instantly.
How It Works
ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. JavaScript implements this via date arithmetic.
Common Use Cases
💡 Business
Reference weeks by number in project planning.
💡 Finance
Identify trading weeks for financial reporting.
💡 Scheduling
Coordinate across time zones using week numbers.
Frequently Asked Questions
Why can a year have 53 weeks?
ISO 8601 allows 53 weeks in years where January 1 falls on Thursday, or in leap years where it falls on Wednesday or Thursday.
Related Tools
Link to This Tool
Copy and paste the code below to link to this tool from your website:
<a href="https://adedx.com/tools/week-number/">Week Number Calculator — AdeDX Free Online Tools</a>