Week Number Finder
Find the ISO week number for any date.
Week Details
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The Definitive Guide to Week Numbers
In our daily lives, we usually stick to dates like "October 15th." But in the business world—especially in manufacturing, logistics, and international trade—dating by Week Number is the standard.
A Week Number corresponds to the specific week of the year, running from Week 1 to Week 52 (or occasionally 53). Using this system eliminates confusion like "early May" vs "mid May" and provides a precise 7-day window for planning.
ISO 8601 Standard
Used by most of Europe, Asia, and international companies.
- Week starts on MONDAY
- Week 1 has the first Thursday
- Standard: 52 or 53 weeks
US System
Primarily used in the United States, Canada, and Mexico.
- Week starts on SUNDAY
- Week 1 contains January 1st
- Can have partial first weeks
Why Use Week Numbers?
Project Sprints
Agile development teams often refer to "Development Week 34" rather than specific calendar dates to plan sprints and deliverables.
Supply Chain
Factories schedule production runs by week. "Production Week 40" is a universal language in manufacturing to target completion windows.
Fiscal Planning
Quarterly reports are often broken down into 13-week standard blocks. Calculating revenue per week is easier than per varying month length.
Frequently Asked Questions
What is an ISO week number?
The ISO week number is part of the ISO 8601 standard used internationally. It defines Week 1 as the first week of the year that contains a Thursday (or the first week with at least 4 days in the new year). ISO weeks always start on Monday.
How is the US week number different from ISO?
In the United States, Canada, and Australia, the week typically starts on Sunday. 'Week 1' is strictly the week containing January 1st, regardless of the day of the week. This often results in different week numbers compared to the ISO system.
What is the current week number?
Our tool automatically highlights the current week number based on today's date when you load the page. Simply look at the default result to see 'What week is it now'.
Can a year have 53 weeks?
Yes. While most years have 52 weeks, years that start on a Thursday (or leap years starting Wednesday) in the ISO system will have 53 weeks. This happens approximately every 5-6 years.
Why do businesses use week numbers?
Week numbers facilitate cross-border planning. Saying 'Delivery in Week 42' removes ambiguity about specific dates or month boundaries, especially for projects spanning multiple months or fiscal quarters.
How do I calculate week number in Excel?
In Excel, you can use the function =ISOWEEKNUM(date) to get the standard ISO week. For the US system, use =WEEKNUM(date, 1).
Does this tool support past and future dates?
Yes. You can select any date from the past or future using the calendar picker to find its corresponding week number, day of year, and quarter.
What is 'Week 52' vs 'Week 1' overlap?
Sometimes, the last days of December (e.g., Dec 29-31) fall into 'Week 1' of the next year if the new year starts on a Thursday. Conversely, early January can be 'Week 52' or '53' of the previous ISO year.
Is this calculator free?
Yes, FreeTools Pro provides this Week Number Finder completely free of charge, with no ads or registration needed.
How are weeks numbered in the Middle East?
In many Middle Eastern countries, the work week starts on Saturday or Sunday. Our tool primarily focuses on the ISO (Monday start) and US (Sunday start) standards most common in international commerce.