Week Number Calculator

How to Use This Calculator

1. Enter a year, month, and day to find its ISO week number, day of week, and position in the year.
2. Use Week to Date to go the other way — find the Monday of any ISO week.
3. Use Year Overview to see how many weeks a year has and when they start.
4. The Professional tier adds fiscal week, academic week, and bi-weekly period numbers.

Formula

Example

Frequently Asked Questions

• What is an ISO week number? ▼
  ISO 8601 week numbers run Monday to Sunday. Week 1 is the week containing the first Thursday of the year. A year has 52 or 53 ISO weeks.
• How does US week numbering differ from ISO? ▼
  US weeks start on Sunday. Week 1 begins on January 1. This can result in partial first and last weeks, and the week numbers are generally 1 higher than ISO for dates early in the year.
• Can a year have 53 weeks? ▼
  Yes — years where January 1 falls on Thursday, or leap years where it falls on Wednesday, have 53 ISO weeks. This happens about 5–6 times per decade.
• What is a fiscal week? ▼
  A fiscal week is counted from the start of your fiscal year (e.g., October 1 for US federal fiscal year). Week 1 = the first week of your fiscal year.
• How do I find the date from a week number? ▼
  Use the "Week to Date" tab — enter an ISO year and week number to get the Monday (start) of that week.

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