Project schedules, manufacturing plans, and international business often reference “week numbers” rather than exact dates — but the ISO 8601 week-numbering rule is easy to get wrong by hand, especially near the start or end of a year. This tool computes it correctly, instantly.
What Is the Week Number Calculator?
Enter any date and instantly see its ISO 8601 week number, ISO week year (which can differ from the calendar year near January 1st or December 31st), day-of-year, weekday, and the exact Monday-through-Sunday date range of that week — alongside a simpler US-style week count for comparison.
How to Use It
- Open the tool at /tools/week-number-calculator.
- Pick a date (it defaults to today).
- Read the ISO week number, week year, and week date range.
Key Features
- Correct ISO 8601 implementation, using the “week containing the year’s first Thursday” rule rather than a naive day-count approximation.
- ISO week year shown separately, since it can differ from the calendar year for dates right at the edges of the year.
- Simple US-style week count included too, for quick comparison against the ISO standard.
- Full week range shown, including the exact Monday start and Sunday end dates.
When to Use This Tool
- Translating a “ship by week 42” deadline into an actual calendar date range.
- Working with European or international teams who commonly reference ISO week numbers.
- Debugging week-number logic in your own code against a known-correct reference implementation.
Try It Now
Open Week Number Calculator directly, or press ⌘K and type the tool name from anywhere on the site.
Part of the Date & Time collection on WebdevToolbox — free, browser-based developer tools with no login required.