Find the ISO 8601 week number (1 through 53) for any date - commonly used in business planning, payroll, and project scheduling.
Week Number Calculator
LiveWeek number
Enter a date
ISO week year
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Uses the ISO 8601 standard: weeks start on Monday, and week 1 is the week containing the year's first Thursday.
How this is calculated
Week 1 = the week containing the year's first Thursday (ISO 8601 standard)
Example
July 19, 2026 falls in Week 29 of 2026.
Step-by-step guide
- Enter a date.
- Read the ISO week number and its associated week-year.
Common mistakes
Assuming week 1 always starts on January 1 - under ISO 8601, week 1 is defined by the first Thursday of the year, so early January days can actually belong to the last week of the previous year.
Mixing ISO week numbering with a Sunday-start week system used by some US calendars - they can give different week numbers for the same date.
Frequently asked questions
Why does ISO week numbering start on Monday?
The ISO 8601 international standard defines Monday as the first day of the week, which is why the "week year" can differ from the calendar year for dates right at the start or end of December/January.
Can a year have 53 weeks?
Yes - most years have 52 weeks, but years where January 1 falls on a Thursday (or a leap year where it falls on a Wednesday) have 53 ISO weeks instead.
Where is week numbering commonly used?
Manufacturing and supply chain schedules, payroll systems, and European business planning often reference week numbers instead of calendar dates.
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