Week Number Calculator
Find the ISO week number for any date, plus how many weeks remain in the year.
Week Number Calculator
Select a date to find its ISO 8601 week number.
Week Number
What This Week Number Calculator Does
This tool finds the ISO 8601 week number for any date you select. It also shows the total number of weeks in that year and how many weeks remain after the selected date. This is useful for project planning, payroll scheduling, and international business communications where week numbers are commonly used.
How to Use the Week Number Calculator
Simply select any date using the date picker. Click the "Find Week Number" button, and the tool instantly returns the ISO week number, the total weeks in that year, and how many weeks are left. No sign-up or account required.
Pro Tip: The ISO week number system is widely used in Europe for invoicing, project deadlines, and manufacturing schedules. Knowing the week number helps teams across different countries align on the same timeline.
About ISO Week Numbers
The ISO 8601 standard defines week numbers where each week starts on Monday, and Week 1 is the week containing the year's first Thursday. This system is used widely in Europe for scheduling, invoicing, and project planning.
ISO Week Rules
| Rule | Detail |
|---|---|
| Week Start | Monday |
| Week 1 Definition | Contains the year's first Thursday |
| Weeks per Year | 52 or 53 (varies by year) |
Why Use ISO Week Numbers?
ISO week numbers are the global standard for numbering weeks. They eliminate confusion between countries that start their week on Sunday versus Monday. They're especially useful for:
- Project management: Tracking milestones across quarters
- Payroll: Standardizing weekly pay periods across teams
- Manufacturing: Planning production schedules by week number
- International business: Aligning deadlines across time zones and countries
Frequently Asked Questions
A year has 53 ISO weeks when January 1st falls on a Thursday, or in leap years when it falls on a Wednesday — this happens roughly once every 5-6 years.
Yes. If January 1st-3rd falls before the year's first Thursday, those days are counted as part of the final week of the previous year under ISO 8601 rules.
ISO weeks start on Monday, while US week numbering typically starts on Sunday. ISO Week 1 is based on the first Thursday, while US week numbers often count January 1st as the start of Week 1 regardless of the day of the week.
Absolutely. This tool uses the official ISO 8601 standard, which is the global reference for week numbering in business, logistics, and project management.