Week Number Calculator

About the Week Number Calculator

The Week Number Calculator determines the ISO 8601 week number for any given date. ISO 8601 defines week numbering as starting on Monday, with week 1 being the week that contains the year's first Thursday (equivalently, the week containing January 4).

Week numbers are widely used in business planning, project management, manufacturing scheduling, and European business communications. "Delivery in week 15" or "report due CW 22" are common references that require knowing the exact week number for a given date.

This calculator implements the full ISO 8601 week-date algorithm, handling the edge cases where dates near year boundaries may belong to week 52 or 53 of the previous year, or week 1 of the following year. For example, December 31 can sometimes fall in week 1 of the next year, and January 1 can fall in week 52 or 53 of the previous year.

When This Page Helps

Week numbers are the standard way to reference time periods in European business, manufacturing, and project management. This calculator gives you the exact ISO 8601 week number for any date, handling the tricky edge cases near year boundaries correctly.

How to Use the Inputs

1. Enter the year of the date.
2. Enter the month (1–12).
3. Enter the day (1–31).
4. The calculator displays the ISO 8601 week number.
5. It also shows the ISO year (which may differ near year boundaries).
6. Use the result for business planning and scheduling references.

Example Calculation

Tips & Best Practices

• ISO weeks always start on Monday and end on Sunday.
• Week 1 is the week containing January 4 (or equivalently, the first Thursday of the year).
• A year has 52 weeks, but some years have 53 (long years).
• Dates in early January may belong to week 52/53 of the previous year.
• Dates in late December may belong to week 1 of the next year.
• European businesses commonly use "CW" (calendar week) notation, e.g., CW 15.

ISO 8601 Week Numbering Explained

The ISO 8601 week numbering system provides an unambiguous way to reference weeks within a year. It is based on three rules: weeks start on Monday, the first week contains the first Thursday of January, and years have either 52 or 53 weeks.

Year Boundary Edge Cases

The most confusing aspect of ISO week numbering is the year boundary. December 29–31 may belong to week 1 of the following year, and January 1–3 may belong to week 52 or 53 of the previous year. This is why the "ISO week year" concept exists—it may differ from the calendar year.

Adoption Worldwide

ISO 8601 week numbering is the standard in most European countries, widely used in international business, and supported by most programming languages and spreadsheet applications. In the US, a different convention (where weeks start on Sunday and week 1 contains January 1) is sometimes used but is less standardized.

Frequently Asked Questions

• What is ISO 8601 week numbering?

  ISO 8601 defines a standard for week numbering where weeks start on Monday, the first week of the year contains the first Thursday of January, and week numbers range from 1 to 52 or 53. This system ensures consistency in international business communication.

• Can January 1 be in the previous year's last week?

  Yes. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week (52 or 53) of the previous year under ISO 8601 rules. For example, January 1, 2027 falls on a Friday, so it's in week 53 of 2026.

• How many weeks does a year have?

  Most years have 52 weeks. Years where January 1 is a Thursday (or Thursday/Friday in a leap year) have 53 weeks. These "long years" occur roughly every 5–7 years.

• Why do weeks start on Monday in ISO 8601?

  The ISO standard reflects the convention in most of the world where Monday is the first day of the work week. This differs from the US convention where Sunday is often considered the first day of the week, but ISO 8601 has been widely adopted for consistency.

• How is week numbering used in business?

  Week numbers provide a compact way to reference time periods. Instead of saying "the week of February 2–8," you can simply say "week 6." This is common in manufacturing schedules, project timelines, delivery planning, and European business communications.

• What is the difference between ISO week year and calendar year?

  The ISO week year can differ from the calendar year for dates near the year boundary. Days in early January might have an ISO week year of the previous year, and days in late December might have an ISO week year of the next year. The calculator shows both.