Week Number Calculator
Enter a date to find its ISO week number and the year associated with that week.
Week numbers are useful in project planning, business reporting, production schedules, school calendars and recurring work plans. This calculator uses the ISO week-date convention.
Choose a date and calculate its ISO week number. ISO weeks begin on Monday and week 1 is the week containing the first Thursday of the year.
Enter the values you want to compare or calculate, then use the result as a practical reference. The calculator runs in your browser and does not require an account.
This tool also fits into a wider set of practical utilities. Depending on what you are trying to calculate or measure next, you can continue with Day of the Week Calculator, Julian Date Calculator, Days in a Month Calculator and Days Between Dates Calculator. For the complete collection, visit the utility tools homepage and choose another tool when your task changes.
How to use this calculator
- Enter your values. Select the dates or numbers required by the calculator.
- Check the inputs. Make sure the dates are in the order and range you intend to calculate.
- Calculate the result. The result appears instantly in the calculator above without submitting a form.
- Use the result. Apply the result to your schedule, deadline, planning or date calculation.
How the calculation works
Enter the values you want to compare or calculate, then use the result as a practical reference. The calculator runs in your browser and does not require an account. The calculation is performed locally in your browser, so the page can return a result immediately after you change the inputs.
Things to know about date calculations
Calendar calculations can differ depending on whether a result is treated as an elapsed interval or an inclusive count. This site uses clear calendar-date rules and explains important assumptions where they affect the result. Business-day calculations on this page exclude weekends but do not automatically remove public holidays.
Frequently asked questions
What is an ISO week number?
An ISO week number identifies a calendar week using the ISO 8601 week-date system, where weeks begin on Monday.
Can a date in January belong to the previous week-year?
Yes. Under ISO week numbering, the first few days of January can belong to the final ISO week of the previous year.