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Week Number Calculator

Find the ISO week number for any date.

Result

Enter values to see your result.

About the Week Number Calculator

Project plans and international business often reference "week 23" rather than a specific date - our Week Number Calculator finds the correct ISO week number for any date.

How It Works

The calculator finds the Thursday of the week containing your entered date (per the ISO 8601 standard, weeks run Monday through Sunday and are numbered by their Thursday), then compares that Thursday's position against the first Thursday of its ISO week year to determine the week number.

Formula & Methodology

ISO 8601 defines a week as running Monday through Sunday, with each week's number determined by which year contains that week's Thursday - this Thursday-anchoring rule is what resolves the ambiguity around New Year's, since the first few days of January can belong to the last week of the prior year, and the last few days of December can belong to week 1 of the following year. The calculation shifts the target date to its week's Thursday, does the same for the year's first Thursday, and counts the number of complete weeks between them to find the week number.

Step-by-Step: Calculating It By Hand

  1. 1Find the Thursday of the calendar week containing the entered date.
  2. 2Find the first Thursday of the ISO week year that Thursday falls in.
  3. 3Count the number of complete weeks between those two Thursdays.
  4. 4Add 1 to that count to get the ISO week number, and note the ISO week year (which may differ from the calendar year near January or December).

Examples

Mid-year date

A date in early June typically falls around ISO week 23, comfortably within its calendar year with no year-boundary ambiguity.

Year-boundary date

A date in the very first days of January can actually belong to week 52 or 53 of the previous ISO week year, rather than week 1, depending on which day of the week January 1st falls on.

Advantages

  • Correctly implements the ISO 8601 standard, including the tricky year-boundary cases
  • Shows both the week number and the ISO week year together
  • Instant, precise calculation for any date
  • Avoids the common off-by-one errors near January and December boundaries

Common Mistakes

  • Assuming week numbering always starts fresh on January 1st, without accounting for the ISO 8601 Thursday-anchoring rule
  • Confusing the ISO week year with the calendar year for dates near the very start or end of the year
  • Using a Sunday-first week convention instead of ISO 8601's Monday-first standard
  • Not accounting for the occasional 53-week year that some years contain

Edge Cases to Watch For

  • A date in very late December can belong to week 1 of the following ISO week year, while a date in very early January can belong to week 52 or 53 of the previous ISO week year.
  • ISO 8601 weeks always start on Monday, unlike some regional conventions that consider Sunday the first day of the week.
  • Some years have 53 ISO weeks instead of the usual 52, depending on which day of the week January 1st falls on and whether it's a leap year.
  • The ISO week year displayed can differ from the calendar year shown on the entered date itself, which is a common source of confusion near year boundaries.

Common Use Cases

  • Referencing project deadlines or milestones by week number
  • International business and manufacturing scheduling that uses ISO week numbers
  • Payroll or reporting periods organized by week number
  • General calendar reference using the ISO 8601 standard
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why might the ISO week number's year differ from the calendar year?

ISO 8601 weeks always start Monday and belong to the year containing that week's Thursday - so the first few days of January can belong to week 52 or 53 of the previous year, and the last few days of December can belong to week 1 of the next year.

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Conclusion

The Thursday-anchoring rule behind ISO week numbers resolves a genuinely tricky edge case around New Year's that a naive week-counting approach gets wrong. Our Day of the Year Counter handles the related but distinct task of finding a date's ordinal position within its calendar year.