Week Number Calculator
Find the ISO week number, day of year, day of week, and quarter for any date.
ISO Week Number
Day of Year
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Day of Week
Quarter
Week Reference
Frequently Asked Questions
What is an ISO week number?
ISO 8601 defines week numbers based on the Gregorian calendar. Week 1 is the week containing the year's first Thursday. Weeks start on Monday. A year has 52 or 53 weeks. Some dates in early January may belong to the previous year's last week, and late December dates may belong to the next year's first week.
Why do week numbers matter in business?
Many industries use ISO week numbers for scheduling, reporting, and planning. Manufacturing uses them for production cycles, logistics for delivery schedules, and accounting for weekly revenue reporting. Saying 'Week 12' is more precise and universally understood than 'third week of March.'
Do all countries start the week on Monday?
ISO 8601 standard starts weeks on Monday, which is followed by most of Europe and Asia. The US, Canada, and Japan traditionally start weeks on Sunday. Some Middle Eastern countries start on Saturday. The ISO standard provides a universal reference regardless of local convention.