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Week Number Calculator Tool

Instantly find the ISO 8601 week number, week of the year, week of the month, and weekday for any date. A reverse mode finds the Monday of a given year and week number.

Last updated: 2026/10/16 Calculator Feedback & bug reports
ISO Week
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Week Number
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ISO Week Year
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Week of Year
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Week of Month
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Weekday
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Weeks in Year
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Week Range
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Monday of the Week
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Sunday of the Week
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Weeks in Year
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How to Use

1

Choose a Mode

Pick "Date → Week No." to get an ISO week number from a date, or "Week No. → Date" to find the dates of a given year and week number.

2

Enter Values

Select a date, or enter a year and week number. Use the "Today" button to fill in the current date with one click.

3

View the Results

Results appear instantly: the ISO week number (year-Www), week of year, week of month, weekday, or the matching Monday and week range.

4

Copy the Results

Click "Copy Result" to copy the full calculation to your clipboard as plain text.

About the Week Number Calculator

A web tool that finds the ISO 8601 week number (year-Www format) for any date. The ISO week is an international standard where weeks start on Monday and week 1 is the week containing the year's first Thursday. It is widely used in business, manufacturing, and scheduling. The tool also shows the week of the year, week of the month, weekday, and supports a reverse calculation from a year and week number back to that week's Monday. All processing happens in your browser and no date is ever sent to a server.

Features

  • Real-time CalculationResults appear instantly as you enter a date or week number. No button press needed.
  • ISO 8601 CompliantFollows the international standard: weeks start on Monday and week 1 contains the first Thursday. Year-crossing weeks are handled correctly.
  • Two-way CalculationConvert a date to a week number, or convert a year and week number back to that week's Monday and Sunday.
  • Privacy ProtectedAll processing happens in your browser. Your dates are never sent to any server.

Use Cases

  • Manage production and logistics plans by week number
  • Align schedules with overseas partners using ISO weeks
  • Weekly project reports and progress tracking
  • Cross-check calendar app or Excel WEEKNUM results
  • Look up "which dates are in week 26?" from a week number

FAQ

What is an ISO week number?
It is the week numbering defined by ISO 8601. Weeks start on Monday, and week 1 is the week that contains the year's first Thursday. It is written as "2026-W26". Around the New Year a week can cross calendar years, so the "ISO week year" of that week may differ from the calendar year.
Does a year have 52 or 53 weeks?
Most years have 52 weeks, but years where January 1 falls on a Thursday (or on a Wednesday in a leap year) have 53 weeks. This tool calculates and shows the total number of weeks for each year automatically.
How is the "week of the month" counted?
Using Monday-starting weeks, the week containing the 1st of the month is week 1, and the target date is numbered accordingly. Because there are several conventions, results may differ by one depending on your organization or system. Treat it as a general guide.
Why does my result differ from Excel's WEEKNUM?
Excel's WEEKNUM defaults to a Sunday-start, January-1 method that differs from ISO 8601. To get ISO-compliant values in Excel use the ISOWEEKNUM function. This tool's ISO week number matches ISOWEEKNUM.
Are my dates stored or sent anywhere?
No. All calculations run locally in your browser (JavaScript). The dates and week numbers you enter are never sent to or stored on any server, so you can use the tool with peace of mind.

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Related Guides

What Is an ISO Week Number? How to Count Weeks Under ISO 8601 A beginner-friendly guide to ISO 8601 week numbers: the rule that weeks start on Monday and the week containing the first Thursday is week 1, how to find the nth week, how it differs from "the nth week of the month," and how to use it in work calendars and project management.