Week Number Calculator: Find the Week Number for Any Date

Our free Week Number Calculator helps you quickly find the ISO week number and US week number for any date. Whether you're planning projects, tracking deadlines, or coordinating schedules, knowing the week number is essential for efficient time management.

ISO Week Number vs US Week Number

There are two common systems for numbering weeks, and they can differ by one or two weeks depending on the date and year:

• ISO Week Number (ISO 8601) - Weeks start on Monday. Week 1 is the week containing the first Thursday of the year. This is the international standard used in most of Europe and many businesses worldwide.
• US Week Number - Weeks start on Sunday. Week 1 is the week containing January 1st. This system is commonly used in the United States.

How Week Numbers Work

ISO Week Numbering Rules

• Weeks always start on Monday and end on Sunday
• Week 01 is the week containing the first Thursday of January
• Week 01 always contains January 4th
• A year has 52 or 53 weeks
• December 29-31 may belong to Week 01 of the next year
• January 1-3 may belong to Week 52 or 53 of the previous year

Why Use Week Numbers?

Week numbers are particularly useful for:

• Project Planning - Schedule tasks and milestones by week
• Business Reporting - Many companies report on a weekly basis
• Manufacturing - Production schedules often use week numbers
• Academic Calendars - Universities track semesters by week
• International Coordination - ISO weeks provide a universal reference

Week Numbers in Different Countries

Most European countries use ISO week numbering (Monday start), while the US traditionally starts weeks on Sunday. When working internationally, always clarify which system you're using to avoid confusion.

Related Date & Calendar Tools

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• Calendar Tool - Interactive calendar with week numbers
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• Time Until Calculator - Count down to any date