Week Number Calculator
Enter any date to find its ISO week number (week starts Monday).
Try it with this example
Enter today's date to get its ISO week number. Week 1 contains the year's first Thursday.
What is this tool?
The Week Number tool converts any date to its ISO week number. In the ISO 8601 standard, weeks run Monday through Sunday, and Week 1 is the week containing the year's first Thursday. That means the first few days of January might belong to Week 52 or 53 of the previous year, and the last days of December might belong to Week 1 of the next year. This tool handles those edge cases so you always get the correct week number.
Week numbers matter for payroll, project planning, retail reporting, and any system that tracks time by week. "Ship by Week 23" or "Report for Week 45" are common. Different regions use different conventions—the US sometimes uses different rules—but ISO week numbers are widely used in Europe and in many software systems. This tool follows ISO 8601 for consistency.
Enter any date; the tool returns the year and week number. Some years have 52 weeks, others 53, depending on how the calendar falls. The tool also helps with the reverse question: "What dates are in Week 27?"—though for that you may need to combine it with a calendar or do a quick trial. For "what week is this date in?", it's direct and accurate.
All processing happens in your browser. No server, no API, no data transmitted. Use it for scheduling, payroll checks, reporting cycles, or anytime you need to align a date with its ISO week quickly.
Bookmark it for planning and reporting workflows.