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Week Number Calculator: ISO Week, Day of Year and Week Dates

This free tool tells you the week number of any date by the ISO standard and the US system. It also shows the day of the year, the quarter and days left, and lists the Monday to Sunday dates of any ISO week.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

Plans, payroll and shipping are often set by week number. Countries use different rules, so it helps to see both the ISO and the US week.

Pick a date, or enter a year and a week to find its dates.

How to Use This Tool

  1. Enter or paste your input in the box or boxes above.
  2. Choose the options, if there are any.
  3. Press the button to run the tool.
  4. Read the result and use the copy button where one is shown.

How the Calculation Works

The ISO week is found by moving to the Thursday of the date's week, taking the year of that Thursday and counting how many full weeks have passed since 1 January of that year. The US week uses Sunday as the first day and counts the week with 1 January as week 1. The day of year counts days since 1 January.

Worked Example

1 January 2021 is in ISO week 53 of 2020. 30 December 2024 is in ISO week 1 of 2025. ISO week 24 of 2025 runs from 9 June to 15 June.

Common Uses

  • Project and sprint planning.
  • Payroll and shipping.
  • School terms.
  • Reading week-based reports.

Tips and Common Mistakes

  • Say which system you use when you share a week number.
  • ISO weeks start on Monday.
  • Week 53 exists in about 1 year in 5.6.
  • Check the first result against a case you already know, so you can trust the tool for the rest.
  • Nothing you type leaves your device. The work is done by your browser, so it also works with a slow connection once the page has loaded.

Next, you may need the Julian date converter, leap year checker, day of the week calculator, date difference calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

Weeks run Monday to Sunday. Week 1 is the week that contains the first Thursday of the year, which is the same as the week with 4 January.
If 1 January falls on Friday, Saturday or Sunday, it belongs to the last week of the previous year in the ISO system.
The US system starts weeks on Sunday and puts 1 January in week 1, so the numbers can differ from ISO by one.
No. Only years that start or end on a Thursday (or a leap year starting on Wednesday) have 53 ISO weeks.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.