📅 Week Number Calculator (ISO & US)
Find the corresponding week number for any date based on the International (ISO 8601) and North American systems.
International Standard (ISO 8601)
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**Week starts on Monday.** Week 1 contains the first Thursday of the year.
North American / Sunday Start
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**Week starts on Sunday.** Week 1 contains January 1st.
Understanding Different Week Number Standards
There is no single global standard for week numbering, which is why business and calendars often use two primary systems: ISO 8601 and the North American standard.
1. ISO 8601 (The International Standard)
This is the standard used across most of Europe and Asia, and often in international business. The core rules are:
- A week always starts on **Monday** and ends on Sunday.
- The first week of the year (Week 01) is the one that contains the first **Thursday** of the new year.
- Consequently, Week 01 may start in the previous calendar year, and the last week (Week 52 or 53) may end in the following calendar year.
2. US / North American Standard
This system is commonly used in the United States and Canada. Its rules are simpler but differ significantly:
- A week always starts on **Sunday** and ends on Saturday.
- The first week of the year (Week 1) is simply the week that contains **January 1st**.
- Partial weeks at the beginning and end of the year are counted as Weeks 1 and 52 (or 53), respectively.